Alexander M. Stolyarov,3 Sylvain Danto,4 John V. Badding,5
Yoel Fink,6 John Ballato,7 and Ayman F. Abouraddy1,*
1CREOL, The College of Optics & Photonics, University of Central Florida, 4000Central Florida Blvd., Orlando, Florida 32816, USA
2Institute of Photonics and Advanced Sensing, School of Chemistry and Physics,ARC Centre of Excellence for Nanoscale BioPhotonics, The University of Adelaide,Adelaide SA 5005, Australia
3MIT Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood St.,Lexington, Massachusetts 02420, USA
4ICMCB/CNRS University of Bordeaux, 87 Avenue du Dr. Schweitzer, 33608 Pessac,France
5Department of Chemistry, Department of Physics & Astronomy, Pennsylvania StateUniversity, University Park, State College, Pennsylvania 16802, USA
6Research Laboratory of Electronics, Massachusetts Institute of Technology,77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA
7Center for Optical Materials Science and Engineering Technologies (COMSET),Department of Materials Science & Engineering, Clemson University, 101 Sikes Ave.,Clemson, South Carolina 29634, USA
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Infrared fibersGuangming Tao, Heike Ebendorff-Heidepriem,Alexander M. Stolyarov, Sylvain Danto, John V. Badding,Yoel Fink, John Ballato, and Ayman F. Abouraddy
1. Introduction
Infrared (IR) light is electromagnetic radiation that starts from the nominal edgeof visible (VIS) light at a wavelength ∼0.7 μm and extends up to ∼1 mm [1–6].Interestingly, there appears to be no consensus on either the nomenclature or thelimits of the various IR spectral divisions, which usually vary according to thedisciplines or applications that exploit the IR [1–6]. For the purposes of thisreview, we refer to the spectral window 0.7–2 μm as the near-IR (NIR), 2–15 μmas the mid-IR (MIR), and 15 μm–1 mm as the far-IR (FIR). The MIR, therefore,covers the important atmospheric windows of 3–5 μm and 8–12 μm [7], whichcomprise critical applications in remote sensing, biophotonics, homeland secu-rity, and minimally invasive medical surgery, to name just a few.
There is currently broad interest in extending optical technologies beyond theVIS and NIR into the less-explored realms of the MIR—driven by multiple op-portunities in novel applications. There is also the general sense that the MIR isthe next frontier for the optics and photonics community, where unexploredterritory awaits to be claimed. The recent interest in the MIR is also partly fuelledby the current availability of MIR semiconductor sources such as quantum cas-cade lasers (QCLs) [8]. Nevertheless, to fully exploit the MIR, a completeswathe of optical technologies must be developed, including coherent narrow-band and broadband sources, sensors, bulk and integrated photonic components,and—critically—IR optical fibers.
In this review, we define IR optical fibers as those that transmit radiation ofwavelengths from ∼2 up to ∼25 μm, which extends across both the MIR andFIR. Fixing precise limits to these spectral windows is not germane to our taskhere, and thus they remain, by necessity, somewhat fuzzy. Such fibers have arich and long history, dating back to the mid-1960s when the first chalcogenide
“A precise border between ‘visible’ and ‘infrared’ cannot be defined,because visual sensation at wavelengths greater than 780 nm is notedfor very bright sources at longer wavelengths. In some applications theinfrared spectrum has also been divided into ‘near,’ ‘middle,’ and ‘far’infrared; however, the borders necessarily vary with the application(e.g. in meteorology, photochemistry, optical design, thermalphysics, etc.).”
The International Commission on Illumination (CIE) Standards [1]
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glass (ChG) fiber made of the IR glass arsenic trisulfide (As2S3) was reported[9]. This early success also highlighted two shortcomings that have been asso-ciated with IR fibers since—high optical losses (>10 dB∕m in the wavelengthrange 2–8 μm [9]) and the lack of mechanical robustness. Dramatic improve-ments have been achieved on both these fronts for IR fibers, which bodes wellfor their potential widespread adoption in the near future.
In contrast to fibers used in the VIS and NIR where only a few materials domi-nate (particularly silica glass), a very broad spectrum of materials has been ex-plored for producing IR fibers—ranging from chalcogenide, tellurite, germanate,and fluoride glasses, to crystalline materials such as yttrium aluminum garnet(YAG), and even—most recently—traditional semiconductors such as silicon,germanium, and zinc selenide. In addition to this surprisingly wide span of IRfiber materials investigated, new concepts are being exploited in engineering IRoptical guidance. For example, the concept of photonic bandgaps (PBGs) under-lies the design of different hollow-core microstructure IR fibers. Furthermore,recent breakthroughs in the compatibility of heterogeneous material familieswith the process of thermal fiber co-drawing have led to the emergence of aunique class of “multimaterial fibers” that may help address some of the per-ennial optical and mechanical shortcomings of IR fibers. Indeed, the implica-tions of the multimaterial-fiber concept may even extend beyond IR fibers tolay the foundation for possible convergence of optics and electronics in a mono-lithic fiber form factor (see the review articles [10,11]).
As such, the field of IR fibers lies at the intersection of materials science, opticalphysics, and manufacturing engineering—and much recent excitement for suchfibers has its origin in the synergy between these communities. The key featureof IR fibers is their ability to transmit wavelengths longer than those afforded byconventional silica glass fibers. Nevertheless, both the optical and mechanicalproperties of IR fibers remain, in general, inferior to conventional silica fibers.For example, most IR fibers have transmission losses in the ∼dB∕m range. Theirusage, therefore, is limited primarily to short-haul applications requiring onlymeters rather than kilometers of fiber lengths, which is sufficient for applicationsin chemical sensing, thermometry, and IR laser power delivery. The repertoire ofapplications that exploit IR fibers is expanding rapidly, and is expected to con-tinue to do so in the next few decades. In this review, we will focus on the recentprogress in developing IR fibers—from the perspective of fabrication and novelmaterials and structures—against a background of previous achievements. Wehope that this venerable but vital field will continue to attract increasing attentionfrom both academia and industry to pave the way for the widespread adoption ofIR fibers.
To provide a broad overview of the topics covered in this review, perhaps it isuseful to start by giving some examples of the IR fibers we will explore. Figure 1depicts a selection of 15 different IR fibers produced over the past decade or so,which highlight the broad range of materials and structures that are currentlybeing investigated. Five distinct families of IR fibers are shown. The first family[Figs. 1(a)–1(c)] comprises step-index fibers where the core and cladding areselected from the same family of soft IR glass, whether fluoride [Fig. 1(a)] [22],tellurite [Fig. 1(b)] [23], or chalcogenide [Fig. 1(c)] glass [24,25]. These threefamilies of glass have been extensively investigated, and fibers produced fromthese glasses have indeed been now commercialized.
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Figure 1
Examples of IR fibers. (a)–(c) Step-index IR fibers in which both the core andcladding belong to the same soft-glass family, in order of increasing long-wave cutoff wavelength: (a) fluoride glass fiber (courtesy of Dr. M.Saad); (b) tellurite glass fiber [12]; and (c) ChG fiber (courtesy of Dr. F.Chenard and IRflex Corporation). (d)–(f) Hybrid step-index fibers thatcombine heterogeneous materials: (d) step-index ChG fiber with a built-inpolymer jacket [13]; (e) ChG core, silica cladding fiber [14]; (f) ChG core,tellurite cladding fiber [15]. (g)–(i) Crystalline-core step-index IR fibers:(g) InSb core, borosilicate glass cladding fiber [16]; (h) ZnSe core, silicacladding fiber [17]; (i) YAG fiber (courtesy of Dr. J. A. Harrington). (j)–(l)Solid-core microstructure IR fibers: (j) all-solid ChG PCF [18]; (k) fluoridePCF (courtesy of Dr. P. Russell); (l) ChG PCF (courtesy of Dr. J. S.Sanghera) [19]. (m)–(o) Hollow-core IR fibers: (m) hollow-core ChG PBGfiber [20]; (n) hollow-core fiber lined with a one-dimensional Bragg structurewith an omnidirectional reflection [21]; (o) hollow-core IR silica fiber withnegative curvature (courtesy Dr. J. C. Knight). (b) and (j): Reprinted withpermission from Refs. [12] and [18], respectively. Copyright 2015 IEEE.(m): Reprinted with permission from [20]. Copyright 2015 Elsevier.
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The second family in Fig. 1 [Figs. 1(d)–1(f)] comprises recently developed hy-brid IR fibers where materials from heterogeneous families of materials are com-bined: chalcogenide glasses with a polymer [Fig. 1(d)], chalcogenide and silicaglasses [Fig. 1(e)], and chalcogenide and tellurite glasses [Fig. 1(f)]. In eachexample, the juxtaposition and integration of two (or more) different materialsin the fiber allows for a unique feature to be exploited for performance enhance-ment, whether mechanical strength or optical functionality.
The third family is that of IR crystalline-core fibers. One such example, YAGfibers [Fig. 1(i)], has been explored for decades, while others have only beenrecently investigated, such as InSb [Fig. 1(g)] and ZnSe [Fig. 1(h)] fibers. Thelatter examples may help usher in a new class of fibers where integration amongoptics, electronics, and optoelectronics could prove feasible [10,26,27].
More recently, a fourth family of solid-core IR photonic crystal fibers (PCFs)have been produced and studied. Examples include all-solid PCFs [Fig. 1(j)] andair-hole-clad PCFs [Figs. 1(k) and 1(l)] realized in either chalcogenide glass orfluoride glass. One may also add to this class so-called suspended-core fibersthat have been realized using a wide range of soft glasses [28,29] and are lendingthemselves to dispersion engineering and nonlinearity enhancement, which arecritical enablers for applications in nonlinear IR wavelength conversion [30,31].
Finally, a fifth family of hollow-core IR fibers is highlighted in Fig. 1, extendingfrom PBG fibers [Fig. 1(m)] with air holes arranged in a two-dimensional latticein the cladding to exploit a strategy that has proven successful in silica PBGfibers, to hollow-core fibers lined with a one-dimensional periodic photonicstructure that provides optical guidance via omnidirectional reflection [32].The latter fiber has been commercialized [33] for minimally invasive surgeryand has helped save or improve thousands of lives to date. A recently emergingparadigm of distinct nature is shown in Fig. 1(o) where a hollow-core silica fiberwith negative curvature exploits an interference effect to guide light in the coreand minimize the overlap with the silica glass.
We are targeting multiple communities in optical and materials science and en-gineering with this review. First, for the sake of those readers whose work wouldbe facilitated by the use of IR fibers, as silica fibers have done for the VIS andNIR, this review brings together the characteristics of various fiber material sys-tems, which will hopefully then help inform the assessment of their choices.Second, for those engaged in IR fiber fabrication, this review brings togetherthe vast repertoire of materials and processing approaches that have been devel-oped in this area. The goal of this review is to provide a comprehensive surveythat illuminates by virtue of the broad coverage and thus enables the cross-fertilization of ideas and concepts.
The paper is organized as follows. Section 2 presents the various classes of op-tical materials that are transparent in the IR. Although it was thought that someof these materials (such as crystalline semiconductors) are incompatible with theconventional process of thermal fiber drawing from a scaled-up preform, recentbreakthroughs have proven this conventional wisdom lacking. Section 3 ad-dresses the criteria for thermal fiber drawing and explains in the processhow these recent unexpected achievements came about. Section 4 provides anoutline of the various fabrication techniques that have been employed to date inproducing IR fibers. The situation here is quite different from that in silica fibers,where one or two strategies [modified chemical vapor deposition (MCVD), and
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stack-and-draw processes] have proven to be silver bullets that provide foralmost all the needs of the silica fiber community. The broad range of hetero-geneous materials of interest in the IR has led by necessity to the developmentof a variety of processing techniques. The choice of any such method isnevertheless constrained by the choice of material(s) to be incorporated inthe fiber, and also by the fiber structure and feature dimension to be realized.
Next, we review the state-of-the-art of IR fibers made of particular glass systems,namely tellurite (Section 5), fluoride (Section 6), and chalcogenide (Section 7)glasses. In Section 7 we highlight the recent work on exploiting the concept ofmultimaterial fibers (here the combination of chalcogenide glasses and poly-mers) to address the lack of mechanical robustness of an IR fiber and to increasethe flexibility in its optical characteristics. Section 8 describes recent additionalwork on multimaterial fibers aiming at IR applications, including hollow-corefibers for high-power IR beam delivery, to thermally drawn crystalline semicon-ductor-core fibers and even single-crystal fiber cores produced by MCVD in apre-existing hollow fiber. In Section 9, we discuss briefly other types of IR fibersthat do not fit into the above classification, some of which have been studied fordecades, while others have emerged over only the past year or two. We thenconclude the paper with an attempt to formulate a roadmap ahead for futuredevelopments.
A few words to the reader regarding what is not contained in this review. IR fibernonlinear optics has developed rapidly over the past decade. The higher nonlin-earity and concomitantly higher chromatic dispersion of most soft IR glasses withrespect to silica provide vast opportunities and introduce new challenges in MIRnonlinear fiber optics. The recent growth spurt in this topic hasmade it impossibleto incorporate those results here, and instead it awaits a review paper dedicated tothat area. Second, we do not address in-depth research on rare-earth-ion doping[34], microstructured optical fibers (MOFs) [35,36], or lasing in IR fibers [37],each sub-topic of which is maturing into a thriving area in its own right.
There is no comprehensive and up-to-date review that covers the broad range ofIR fibers provided here. However, the book by Harrington [38] has been astandard reference on IR fibers, especially for crystalline fibers, and we havebenefited from it in preparing this review. We hope that the additional progressachieved in this field over the past decade since its publication is well-represented here.
2. Infrared Materials
Optical fibers are typically drawn from glassy materials such as inorganicglasses [39] and polymers [40]. In the NIR, high-purity silica glass has provento be an ideal material for optical transmission [Fig. 2(a)] and is readily drawninto a fiber. In moving to longer wavelengths in the IR where silica is no longer aviable option, other materials—glassy or otherwise—need to be considered.Indeed, despite the fact that crystalline materials (such as Si and ZnSe) arenot suitable for direct thermal fiber drawing [10], their favorable IR opticalproperties—in addition to their electronic and optoelectronic characteristics—have invited recent attempts at incorporating them into hybrid multimaterial fi-bers. Such fibers combine both crystalline and glassy components as the coreand cladding, respectively, and offer multiple functionalities integrated mono-lithically in the fiber [10]. We will therefore discuss the IR optical transmission
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Figure 2
Optical transmission of various IR materials. (a) Materials that transmit from theVIS to the MIR (the sample thickness is in parentheses): crown glass (10 mm)[55], IR fused silica glass (1 mm) [55], and sapphire (3 mm) [55]. (b) Materialsthat transmit from the MIR to the FIR (the sample thickness is in parentheses):ZnSe (3 mm), Si (6 mm), Ge (1 mm), As2S3 (ChG; 10 mm) [55], ZBLAN fluo-ride glass (FG; 2 mm) [56], and tellurite glass (TeG; 2 mm) [57]. The atmos-pheric transmission window is shown in the background. Solid lines correspondto the transmission of glassy (G) materials and dashed–dotted lines to crystalline(C) materials.
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of glassy and crystalline materials alike in anticipation of the subsequentsections of this review.
Figure 2 depicts the optical transmission spectra for a variety of glassy andcrystalline IR materials. Note that, in general, the transmission spectrum of amillimeters-thick bulk differs from that of a meters-long fiber of the samematerial—the cut-off wavelength of the latter is significantly lower and thetransparency window is narrower, as highlighted in Table 1 [7,13,41–49].Additionally, differences between the measured transmission windows of bulkand fiber with the same length arise mainly from defective bonds introducedduring the glass-melting and thermal-drawing processes [50]. Commonly usedoptical crown glass [51] transmits light only up to 2 μm even in thin samples[Fig. 2(a)]. Traditional single-mode silica fibers provide a low-loss transmissionwindow that reaches only 2.1 μm, while 1-mm-thick bulk silica transmits lightup to 3.5 μm [Fig. 2(a)]. Indeed, loss in silica fibers at 2.94 μm is estimated to be800 dB/m, 4 × 106 times higher than that at 1.5 μm [38], which may neverthelessallow for very short lengths of silica fiber to be used in the 3–3.5 μm region [52].Absorption in silica rises dramatically to thousands of dB/m beyond 3.5 μm [53].Recently developed hollow-core microstructure fibers aim at eliminating thematerial absorption of silica beyond 2.1 μm using negative-curvature fiber struc-tures [54]. The loss at 2.94 μm may be reduced to 0.06 dB/m (1.1 dB/m) fornegative-curvature (positive-curvature) fiber structures. The limit set by absorp-tion in silica remains insurmountable in the MIR and FIR.
Crystalline sapphire is used in optical windows since its transmission extendsfrom the UV to the MIR [0.15–5.5 μm; see Fig. 2(a)] and is also extraordinarilyscratch-resistant—more so than any other optical material [58]. Although sap-phire is crystalline and thus cannot be thermally drawn directly, neverthelessapproaches to produce sapphire fibers have been developed [38]. The maindrawback of sapphire as a fiber material is the difficulty to identify a thermallycompatible glassy cladding material [59–61]. It is conceivable that the approachused for crystalline YAG fibers—dipping the bare crystal fiber into a soft glassliquid to add a cladding [62]—may be applicable.
In Fig. 2(b) we examine materials that transmit light farther in the MIR.Generally speaking, IR glasses can be broadly classified into three groups:heavy-metal oxides, halides (fluorides and chlorides), and chalcogenides.The most popular IR heavy-metal oxide glasses are tellurite and (lead-) germa-nate glasses. Tellurite glasses are characterized by a wider transmission windowthan silicates that extends into the MIR, a lower phonon energy than silica, andhigher linear and nonlinear refractive indices compared to other oxide glass sys-tems [63], while remaining more stable than fluorides (see Section 5). These
Table 1. Typical Optical Parameters of Selected IR Glassesa
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promising features have led to the commercialization of IR tellurite fibers [23].Fluoride glasses, including ZBLAN glasses, constitute a class of non-oxide op-tical glasses composed of fluorides of various metals. The first fluoride glass wasdiscovered by Poulain et al. at Université de Rennes (France) in 1970s [41]. Atypical fluoride glass has less thermal and chemical stability than other IRglasses, but has a wider transmission window compared to oxide glasses (thoughnarrower than chalcogenide glasses). While a large number of multicomponentfluoride glass compositions have been synthesized, only a few have been drawninto fibers [64] (see Section 6). To date, chloride glasses have not proven to besufficiently stable thermally for most applications [65].
Figure 2(b) shows the typical optical transmission of a chalcogenide glass(ChG). ChGs have found many applications in IR lens molding [66], nonlinearoptics [67], phase-change memories [68,69], sensors [70], integrated photonics[71], and optical fibers [72]. Typically, sulfur (S)-based, selenium (Se)-based,and tellurium (Te)-based ChG bulk material transmit 0.6–12 μm, 1–15 μm, and1.5–20 μm, respectively. The transparency window of ChGs makes them attrac-tive for use in IR fibers (see Section 7) and as waveguides. Currently, there arecommercially available IR ChG fibers produced by the double-crucible method[24,25] (see Section 4.2).
An intriguing class of IR materials—and one that has attracted attention onlyvery recently to its potential as an IR fiber material—is crystalline semiconduc-tors. In addition to the favorable IR optical properties of Si, Ge, and other com-pound semiconductors, they also display excellent electronic and optoelectronicfunctionalities that may potentially be integrated with the optical functionality ofa fiber. Therefore, incorporating such materials in a fiber may allow for con-structing new active fiber devices [26,73,74].
The attraction of Si as an IR material is that it offers wide transparency in the IRand strong nonlinear optical effects. Silicon not only has broad IR transmissionof 1.2–10 μm [Fig. 1(b)], but it also transmits 48–100 μm radiation [75], whichhas led recently to increased interest in silicon-based MIR photonic devices [76][77]. Crystalline Ge transmits across the whole atmospheric spectral window,and thus has been utilized in manufacturing lenses for IR-imaging systems sincethe 1990s (note, however, that the transparency of Ge is highly temperature-dependent, becoming opaque when heated). Both Si and Ge possess strongthird-order nonlinear optical coefficients. Indeed, two-photon absorption, whichis a limiting factor for nonlinear optical processes in the NIR, vanishes at longerwavelengths where the energy of two photons is not enough for a band-to-bandtransition [78]. Therefore, efficient nonlinear optical effects and devices atlonger wavelengths are expected in the IR. There are current approaches todrawing Ge- and Si-core fibers with silica or borosilicate glass cladding [79–81].We discuss in Section 8 the progress so far in utilizing Si and Ge (among othercrystalline semiconductors) as IR fiber materials.
Finally, ZnSe—a critical IR II–VI optoelectronic crystal [82–84]—transmitslight from the VIS to the FIR (up to ∼21.5 μm in 1-mm-thick samples [85]and up to ∼14 μm in 12-mm-thick samples). In addition to its usefulness in con-structing bulk optical components (such as lenses) for IR systems [86], ZnSe is auseful IR laser host medium [87]. Thermal drawing of ZnSe into a fiber has notbeen demonstrated to date, but an alternate approach [17] has succeeded in pro-ducing single-crystal ZnSe inside a hollow silica fiber. Recent work on in-fiber
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chemical synthesis has provided a new route to ZnSe fiber cores, where the crys-talline ZnSe material is formed during the drawing process [88,89].
In conclusion, a wide range of amorphous and crystalline materials exist withbroad IR transparency. Their suitability for thermal fiber drawing is the subjectof the next section.
3. Criteria for Drawing an Infrared Fiber
The most common approach to producing an optical fiber is thermal drawingfrom a macroscopic scaled-up model—essentially bulk material—called a “pre-form.” The preform is typically an axially symmetric cylindrical rod with diam-eter of the order of millimeters to a few centimeters and whose volumedetermines the fiber length produced. The internal transverse structure of thepreform is designed to correspond to that of the desired fiber cross section.The preform is heated in a furnace until the material softens and plasticallydeforms under axial stress, thereby enabling thermal drawing into an extendedfiber.
From the perspective of thermal drawing, the temperature (T ) dependence ofviscosity (η) is the most important physical characteristic of the materials com-bined in the preform. During drawing, whereupon the heated preform diameter isreduced to that of the fiber and the length concomitantly increases, the viscositymust be held relatively low—typically in the range from 104 to 107 Poise. Suchan approach is not materials agnostic. Indeed, given the broad range of IR ma-terials described in Section 2, which have quite diverse thermal and mechanicalproperties, thermal drawing is not suitable for all IR materials. Careful consid-eration of the materials’ thermomechanical characteristics is necessary, particu-larly the temperatures at which the materials deform plastically, crystallize, andultimately degrade. Consequently, alternative fiber fabrication approaches havebeen developed. We review these methodologies in Section 4 and focus here onthermal drawing only.
To appreciate the impact of viscosity on thermal drawing, let us first considertwo limiting scenarios. On one hand, too high of a viscosity during drawingprevents the material’s plastic flow and results in the fiber breaking. On the otherhand, too low of a viscosity during drawing may potentially initiate capillaryfluid instabilities, leading to a departure from the intended axial translationalsymmetry along the fiber [90,91]. This is particularly critical when submicrom-eter-scale highly curved surfaces are involved since surface energy may domi-nate over inertial viscous forces.
Figure 3(a) shows the viscosities of some materials highlighted in Section 2 fortheir IR transparency. Two distinct classes of materials emerge with respect tothe thermal dependence of their viscosity. First, glassy materials, which lacklong-range order, are characterized by a continuous and monotonic drop in vis-cosity with increasing temperature. Such materials are amenable to thermaldrawing into a fiber; e.g., silica, As2S3, FG, TeG, and PEI polymer in Fig. 3(a).Second, crystalline materials undergo an abrupt phase change upon reachingtheir melting temperature—transforming from a solid directly to a low-viscosityliquid with little subsequent change in viscosity upon further increase in temper-ature; e.g., Si, Ge, and InSb in Fig. 3(a). Such materials—standing alone—arethus not amenable to thermal drawing into a fiber from bulk. Nevertheless,
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the emerging concept of multimaterial fibers has paved the way to thermallydrawing crystalline materials into a fiber by relying on an amorphous scaffoldthat may be co-drawn along with the crystalline material to provide mechanicalsupport (see Section 8 and Ref. [10]).
Glassy materials are particularly suitable for thermal drawing since they may beplastically deformed in a soft or supercooled state between the solid and liquidstates that may be gradually reached by heating. This applies to almost all IRglasses and also to thermoplastic polymers. As a rule of thumb, the stable rangeof viscosity for thermally drawing a material into a fiber is 104–107 Poise. It isthus desirable for this viscosity range to be achieved at temperatures that arehigher than the glass transition temperature Tg but lower than the crystallizationtemperature Tx. For example, if the viscosity of a glassy material at Tx is largerthan 107 Poise, this material is not suitable for thermal drawing. Additionally,materials with small values of ΔT � Tx − Tg, particularly ΔT < 100°C, gener-ally are less stably drawn into a fiber.
Thermal drawing of soft glass fibers is well known to be more challenging thandrawing silica fibers, particularly with respect to their sensitivity to minutechanges in the drawing temperature. Figure 3(a) provides a clue to the reason:the change of viscosity with T in the thermal-drawing region for silica is very
gradual (dηdT is relatively small), while the viscosity–temperature slope is very
sharp for most IR glasses (dηdT is relatively large). Consequently, a change in Tof only a few degrees may lead to a significant change in viscosity and, hence,in the drawing conditions for IR glasses, while a similar change would have al-
most no impact on silica. For example, dηdT jη�106≅ 1.5 × 104 Poise∕°Cfor silica at
1925°C, while dηdT jη�106
≅ 8.2 × 105, 7.4 × 105, and 1.6 × 105 Poise∕°C for
Figure 3
0.2 0.4 0.6 0.8 1.0
Tg/T
Silica ChG FG TeG PEI
0 500 1000 1500 2000 2500
-2
0
2
4
6
8
10
12
InSb Si Ge PEI lo
g (v
isco
sity
) in
Poi
se
Temperature T (in oC)
Silica ChG FG TeG
Thermal-drawing viscosity
Thermal-drawing viscosity
(a) (b)
Viscosity curves of selected materials. (a) Viscosity of some common IR ma-terials (see Fig. 2) versus temperature T in °C (data of PEI are measured atCREOL/UCF, the rest of the data are adapted from Fig. 2 in Ref. [10]).ChG, As2S3; FG, the fluoride glass ZBLAN; TeG, the tellurite glass75TeO2–20ZnO–5Na2O; PEI, the polymer polyetherimide. (b) Viscosity ofsome common glassy IR materials versus Tg∕T selected from (a) silica(Tg � 1215°C), ChG (Tg � 187°C), FG (Tg � 260°C), TeG (Tg � 299°C),and PEI (Tg � 216°C). The thermal-drawing viscosity region is highlightedin the background. Solid lines correspond to glassy materials, and dashed–dottedlines to crystalline materials.
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tellurite glass at 378°C, fluoride glass at 314°C, and chalcogenide glass at 300°Cin Fig. 3(a). This fact is further highlighted in Fig. 3(b) where we plot viscosityversus a normalized inverse temperature Tg∕T of the same materials in Fig. 3(a).We note that the curves for silica andAs2S3 are almost identical. Nevertheless, thelower Tg for As2S3 compared to silica (187°C–1215°C, respectively) results inthe drawing process having a higher sensitivity with regards to temperaturechanges in a realistic setting. Assuming one has access to an ideal furnace,one in which the temperature is perfectly controlled without fluctuations, anymaterial that has viscosity located in the thermal-drawing range may be, in prin-ciple, drawn into a fiber. For more details of studies on thermal drawing, seeRefs. [92–101].
Table 2 [24,42,102–109] lists some of the critical thermomechanical character-istics that impact thermal fiber drawing for several optical materials, includingsilica, Schott SF6, the fluoride glass ZBLAN, the chalcogenide glass As2S3,polymethyl methacrylate (PMMA), and polyethersulfone (PES). We also listair in Table 2 for comparison. For better control over the preform-to-fiber trans-formation, it is preferable for the materials to have low thermal expansion toavoid structural deformation. The low thermal expansion of silica with respectto IR glasses is behind the more precise control over the structure of silica MOFs[35,36] compared to that achieved in IR MOFs [12,20,28,110,111]. A large spe-cific heat necessitates a longer heating time to soften the material, but renders thematerial less sensitive to temperature fluctuations during drawing and the proc-ess thus more stable. A large thermal conductivity coefficient allows for the useof a narrower heating zone and thus faster drawing speed with a large reductionin diameter from the preform to fiber.
In addition to viscosity, there are multiple other parameters characterizing amaterial that impact the approach chosen for preform preparation, its thermaldrawability into a fiber, and the handling of the drawn fiber. For example,the bulk mechanical characteristics may dictate the choice of nonthermalprocessing routes, such as drilling or stacking, to prepare the preform (seeSection 4). Furthermore, convenient handling and storage of IR fibers necessi-tates the mechanical and chemical stability of the fiber materials. This is notalways the case for IR materials; e.g., fluoride glass is prone to absorb moisture.Consequently, a polymer coating is essential for protection and increasing themechanical robustness of IR fibers. Additional characteristics that impact theproperties of the drawn fibers include the fatigue, strength, bending, hardness,and aging of the materials.
An important factor to consider when drawing an IR material into a fiber is thatthe preform is suspended in free space inside a furnace. The glass is heated upby absorption of radiation from the heat source and conduction from the
Table 2. Comparison for Silica, SF6 Glass, Fluoride Glass ZBLAN, ChalcogenideGlass As2S3, Polymer PMMA, Polymer PES, and Aira
a Tg, glass transition temperature; TEC, thermal expansion coefficient; σ, specific heatcapacity; C, thermal conductivity.
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surrounding (gaseous) environment. In the case of IR-transmitting glasses, theabsorption of radiation may potentially be low, which adds difficulties in heatingup the material.
Finally, recent advances in fiber manufacturing have started to highlight the po-tential role of fluid instabilities in the fiber-drawing process itself [91,112–114].The first intimation of this role was detected in an investigation of the origin ofunexpectedly high optical transmission losses in hollow-core PBG fibers. Acareful study of the free silica surface lining the hollow core revealed surfaceroughness compatible with frozen surface fluctuations having wavelengths pre-dicted by the equipartition theorem. It is theorized that thermodynamicallydriven fluctuations arise at the free silica–air surface during drawing due tosurface tension while the material viscosity is low. These fluctuations are frozenon the silica surface upon cooling, and the surface roughness results in unwantedoptical scattering. This phenomenon does not occur in traditional silica step-index fibers since the surface energy at the core–cladding interface is negligible.
In multimaterial IR fibers, the surface energy at the heterogeneous interfacesmay lead to new physical phenomena as a consequence of fluid instabilities.Two examples have been reported to date: breakup of a thin cylindrical film ofan IR glass embedded in a thermoplastic polymer into an azimuthal array ofaxially intact filaments [115,116], and the axial breakup of a cylindrical IR glasscore embedded in a polymer cladding into a necklace of spheres [112–114].
4. Infrared Fiber Fabrication Methodologies
With the above discussion of the thermomechanical properties of candidate ma-terials for IR fibers in mind, we now proceed to a more detailed description of IRfiber fabrication. The situation here is quite distinct when compared to silicafibers where two standard approaches have emerged: MCVD for step-index fi-bers and the stack-and-draw approach for MOFs. The vast richness of availableIR materials with a varied assortment of thermomechanical characteristics hasresulted in the proliferation of a multiplicity of heterogeneous fabricationapproaches that have yet to be standardized. We classify here the availableIR fiber fabrication methodologies into two broad strategies: preform-to-fiberand non-preform-based approaches.
Preform-to-fiber approaches produce IR fibers from a macroscopic-scale bulkmaterial, the preform, while non-preform-based approaches, as the name indi-cates, produce IR fibers without the preform intermediary. Instead, the fiber isproduced directly from the melt as in the double-crucible method [117], from abillet or disk as in the hot-extrusion method [118,119], or by the cladded high-pressure microfluidic chemical deposition (HPMCD) method [74].
Not every IR material has been traditionally amenable to thermal fiberdrawing. Nevertheless, recent advances in multimaterial fibers, micro-and nano-structured fibers, and fibers produced by nontraditionalfabrication routes have dramatically increased the portfolio ofmaterials that are currently used in IR fibers.
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In general, there are two technological barriers that have plagued IR fibers:achieving low optical loss and achieving high mechanical strength or robustness.The optical loss in IR fibers stems from intrinsic absorption in the bulk materials(Section 2) and imperfections in the fiber structure that are introduced duringpreform fabrication and fiber drawing. The lack of mechanical robustness stemsfrom the intrinsic mechanical properties of soft IR glasses and crystalline ma-terials, which are typically brittle. Progress in materials purification has im-proved both IR fiber loss and strength by reducing glass-matrix defects inthe fiber [120,121]. Furthermore, recent efforts in the area of multimaterial fibers(see Sections 7 and 8) have led to an altogether different approach for increasingIR fiber robustness. For example, an IR-transparent but mechanically brittleChG step-index structure may be integrated with a thick robust built-in polymerjacket to offer superior mechanical support while not participating in the opticalfunctionality [10]. Further research is needed to standardize these recentadvances.
4.1. Preform-to-Fiber Approaches
The preform-to-fiber approach for IR fibers is similar overall to that employed inproducing silica fibers: a macroscopic scaled-up preform is prepared from bulkmaterials, followed by continuous thermal drawing of the preform into an ex-tended fiber (Section 3). Thermal drawing then reduces the transverse dimensionwhile uniformly elongating it axially. The preform is fed into a furnace that soft-ens the material, after which gravity or an external force pulls the molten gob atthe preform tip until it stretches into a thin strand whose diameter is monitoredwith a gauge. This relatively simple fabrication process is behind the thousandsof kilometers of silica optical fibers used in the communications networks thatspan the globe today [39]. Polymer coatings are typically applied to the fibersurface for protection. An alternative approach that is suitable for some IR ma-terials is to incorporate a built-in polymer jacket that is co-drawn with the fiber,which requires that the polymer chosen be thermomechanically compatible withthe IR fiber material, such as a ChG [10,122,123] or a ZBLAN fluorideglass [124].
4.1a. Step-Index Fibers
An archetypical fiber structure is the step-index (core–cladding) geometry.While producing a preform that draws into such a structure in silica is stand-ardized via MCVD, producing an IR step-index preform is challenging and ahost of processes have thus been developed, as outlined in Fig. 4. The mostutilized strategy is called, for obvious reasons, the “rod-in-tube” approach,wherein a cylindrical rod of the higher-refractive-index core material is preparedseparately from a hollow tube of the lower-refractive-index cladding material.
There is a multiplicity of general fabrication strategies that yield fibersstarting from raw materials. For any given IR material, there could bemultiple approaches that yield an IR fiber. The choice of any givenprocess is governed by the characteristics of the material, thecompatibility between the materials combined in the same fiber,and the target structure.
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Alternatively, the preform may be produced in a single step by extruding a struc-tured billet.
I. Rod-in-tube approach for step-index fibers. Figure 4 depicts variations on therod-in-tube method that have been used to produce step-index IR fiber preforms.The core rod [Fig. 4(a)] may be produced by melt casting, Fig. 4(a-1); thermallydrawing a cane from a larger rod, Fig. 4(a-2) [35,36]; extrusion, Fig. 4(a-3)[110,125]; or hot-pressing from a powder, Fig. 4(a-4). The cladding tube[Fig. 4(b)], on the other hand, may be produced by casting, Fig. 4(b-1) [126];drilling, Fig. 4(b-2) [122,127,128]; extrusion, Fig. 4(b-3) [125,129]; or rota-tional casting, Fig. 4(b-4) [130,131].
Theoretically, permutations of pairs of individual processes shown in Fig. 4would lead to 24 possible methods to produce an IR step-index fiber preform.However, for any specific choice of core and cladding materials, only a limitedsubset of processes may be relevant. For example, casting- and drilling-based
Figure 4
General methodologies for producing IR step-index preforms. (a) Core rodpreparation via (a-1) casting, (a-2) thermal drawing, (a-3) extrusion, and(a-4) hot press. (b) Cladding tube preparation via (b-1) casting, (b-2) drilling,(b-3) extrusion, or (b-4) rotational casting. After preparing the rod and tube inthe solid state, the rod is inserted into the tube to form a preform assembly.(c) Rod-in-tube casting.
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methods are not appropriate for crystalline materials, and hot-pressing might bethe only alternative in Fig. 4(a) that is appropriate if the crystalline material is notavailable in the form of a rod. Toxic materials or glasses with high vapor pres-sure in the liquid sate are ill-suited for use in casting methods. MCVD providesexquisite control over the refractive index of silica preforms. In comparison,melt-casting methods do not offer precise control over the refractive index, sincethe glass melts possess vapor pressures that may lead to loss of componentsthrough volatilization. This may cause changes in the glass composition, leadingto deviations in the intended refractive index. Finally, the dimensional tolerancesbetween the rod and tube result in an unavoidable gap at the core/cladding inter-face, which ultimately contributes to optical scattering due to bubble formationand glass soot deposition.
A.Rod preparation
Melt-casting. Raw materials are typically melted in a closed, evacuatedcontainer [Fig. 4(a-1)], but materials with low vapor pressure and that do notoxidize may be melt-cast in an open container. This process has been appliedto most soft glasses, including tellurite, fluoride, and chalcogenide glasses.
Thermal-drawing. A small-diameter rod or “cane”may be thermally drawn froma pre-existing larger-diameter rod [Fig. 4(a-2)], as long as the thermal drawingprocess does not significantly decrease the purity of the material.
Extrusion. Extrusion is a well-established technique to produce axially symmet-ric rods with complex cross-sectional structure from bulk material, and has beenused to produce rods from most IR glasses [132,133] and even crystalline ma-terials [134]. As shown in Fig. 4(a-3), bulk material (usually called a “billet”) isplaced in a metal tube (sleeve) and is heated up to the material softening temper-ature. A force is then applied to push the softened material through a small-dimension die to produce a tube with the desired dimension and structure.
Hot-pressing. Starting from powderized raw material, as is the case for mostsemiconductor materials, hot-pressing is a viable option to condense powdersinto a solid rod under high pressure at a suitable temperature, as shown inFig. 4(a-4).
B. Tube preparation
Melt-casting. Preparation of a hollow cladding tube via melt-casting[Fig. 4(b-1)] bears similarity to the process used to prepare a core rod[Fig. 4(a-1)] with one crucial difference. Hot glass liquid is first poured intoan open container that is kept at a low temperature, and the inner hot glass thatremains in a liquid state is poured out—leaving behind glass material taking theform of a bottle-like inner tank. Post-machining and polishing then yield thedesired glass tube. This process has been used for some IR glasses with lowvapor pressure in the liquid state, such as tellurite and fluoride glasses, butnot for most of chalcogenide glasses.
Drilling. Either mechanical or ultrasonic drilling [Fig. 4(b-2)] may be applied tomost IR glasses to produce a tube, including tellurite, fluoride, and chalcogenideglasses. Further machining and polishing processes are typically necessary toobtain an optical-quality inner surface. Indeed, beyond simple tubes, drillinghas also been used to prepare MOF preforms [135].
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Extrusion. By modifying the die structure, a tube can be extruded from a solidbillet [Fig. 4(b-3)] instead of a rod. In general, extrusion is quite versatile withregards to the materials that may be exploited or structures produced, the latter ofwhich may be engineered through judicious design of the die or the billet.Extrusion may thus be used to produce MOF preforms [136] and even multi-material fiber preforms [10].
Rotational casting. Rotational casting has been exploited in producing tubesof tellurite, fluoride, and chalcogenide glasses. In this approach, raw glassmaterial is sealed in a silica or metal cylindrical container mounted in a hori-zontal lathe. The sealed container is rotated at a high speed (∼ several thousandrpm) while maintained above the material melting temperature. Centrifugalforce shapes the glass liquid into a tube, followed by a careful cooling process[130].
C. Rod-in-tube assembly
With the preparation of the desired rod and tube, a rod-in-tube assembly isformed by simply inserting the rod into the tube. The result is a step-index fiberpreform en route to thermal drawing.
II. Rod-in-tube casting. An alternative approach to preparing the rod-in-tubeassembly that starts from a pre-existing tube relies on directly casting therod into the tube. Figure 4(c) shows two examples, where the tube is insertedinto the core liquid or the core liquid is poured into the tube to form a rod-in-tubeassembly upon cooling. This strategy particularly helps to obtain an intermediatepreform with a small core-to-cladding ratio [126] for single-mode fiber, in ad-dition to improving the quality of the core–cladding interface. For glasses havinga high vapor pressure in the liquid state, such as chalcogenides, complex setupshave been designed to produce step-index fiber preforms [126,130].
III. Multimaterial coextrusion for step-index fibers. Structured fiber preformsmay be obtained via extrusion in one step by structuring the billet itself. Forexample, using a vertically stacked billet consisting of multiple discs, the hori-zontal interfaces are converted into a vertically nested structure [Fig. 5(a)][13,137–144]. This strategy can therefore yield step-index structures from a bil-let consisting of two disks [137,139] or 1D multilayer structured preforms from abillet comprising more than two disks [140–142,144]. This strategy can betraced back to efforts by Itoh et al., reported in 1994 [139], to extrude astep-index preform made of fluorozirco-aluminate glass from two stacked disks.Subsequently, research groups at Nottingham University [144], RutgersUniversity [141], and Southampton University [142,143] reported similar ap-proaches to produce step-index and 1D MOF preforms. An alternative approachreported by the Université de Rennes 1 relies on a custom system involving aseries of in situ vacuum distillations followed by casting of the preform throughsequential rotation [145].
Alternatively, a rod-in-tube assembly may be used as the billet [122]; Fig. 5(b).This method was exploited recently to produce robust step-index Te-basedchalcogenide fibers for long-wave IR transmission using a multimaterial billet.The compression of the structure occurring during extrusion helps eliminate anypotential gaps at the core–cladding interface in this approach.
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4.1b. IR Microstructure Fibers
MOFs provide a large degree of controllability over the fiber performancethrough structural design while constructing the fiber from a single materialby the incorporation of air holes in the cladding. This approach has provenparticularly valuable in silica in the VIS and NIR [35,36]. A unique aspectin the IR is the large span of refractive indices of IR materials (Table 1).This feature offers the potential for constructing all-solid MOFs while maintain-ing a large refractive index contrast. Several methods have been utilized to datein fabricating IR MOF preforms, including the stack-and-draw method [35,36],thin-film rolling [11], extrusion [136], MCVD [146], drilling [135,147], andcasting [148].
Stack-and-draw. The stack-and-draw approach has been used extensively in pre-paring silica MOFs, both PCFs and PBG fibers [35,36,149]. The first demon-stration of the stack-and-draw method to produce an optical fiber may be tracedback to Bell Labs in 1974 [150], where a fiber containing a hanging core sur-rounded by air was produced. Rods, tubes, or plates from a single material ormultiple materials may be assembled into a preform [Fig. 6(a)]. Multiple stack-and-draw steps may be applied recursively to reach the required dimensions andattain complex transverse structures. However, stacking raises challenges forreproducibility and is less suited to fragile glasses. Nevertheless, the stack-and-draw method has been used to produce the first hollow-core chalcogenidePBG fiber [20], but low-loss IR transmission has not yet been confirmed. Thestack-and-draw method has also been used to produce all-solid MOFs [18,151].
Thin-film rolling. Polymers may be incorporated into an IR fiber—typically toimpart mechanical robustness—using any of the above three approaches. Aunique process involves rolling a thin polymer film around a rod followedby thermal consolidation under vacuum above the glass transition temperatureof the constituent materials to allow the individual films to fuse [Fig. 6(b)]. Thepolymer film can be replaced by a bilayer film (polymer film with depositedmaterial), resulting in a 1D multilayer structure. This process is the basis forfabricating hollow-core multimaterial PBG fibers with an all-solid photonicstructure lining the core that provides an omnidirectional bandgap (seeSection 8) [21].
Extrusion. Extrusion can be used to produce MOF fiber preforms from softglasses with complex structures [141,142]. Figure 6(c) depicts the extrusion
Figure 5
)b()a(
Sleeve
Billet
Die
Preform
Multimaterial coextrusion strategies for IR fiber preforms. (a) Multimaterialstacked coextrusion and (b) multimaterial rod-in-tube coextrusion.
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of a MOF preform schematically. Compared with the stack-and-draw, drilling,and casting approaches, extrusion involves a single step to produce the preformdespite the complexity of the transverse structure. Kiang et al. [152] reported thefirst MOF preform fabricated by extrusion. Ebendorff-Heidepriem and Monrohave reported significant advances in preform extrusion and die design for thefabrication of complex-structured preforms using soft glasses [136].
Other approaches. Drilling produces tubes that might be used either as cladding[Fig. 4(b-2)] or canes for the stack-and-draw method [Fig. 6(a)], and may evenbe used to produce IR MOF preforms directly [135,148,153]. However, drillingis limited to short preform lengths and necessitates polishing to reduce surfaceroughness and contamination. Casting techniques may be used to produce rods,tubes, and rod-in-tube assemblies for step-index fiber preforms [Fig. 4], and mayadditionally be used to produce MOF preforms from soft glass [148]. To date,casting of preforms with a large number of transverse features has been achievedonly by using sol-gel and in situ polymerization techniques. In the context of IRfibers, an exception has been the demonstration of chalcogenide MOF preformsproduced from a complex sealed silica casting setup [148].
4.2. Non-Preform-Based Approaches
A particularly important process that has proven useful for producing high-quality IR fibers—initially from fluoride glasses in the early 1980s [117]and subsequently chalcogenide glasses—is the double-crucible fiber drawingapproach, the principle of which is shown in Fig. 7(a) [154]. A system oftwo crucibles is assembled: an inner quartz crucible concentrically positionedwithin an outer quartz crucible, with each connected individually to an inertgas source. The core and cladding glasses are placed in the inner and outer cru-cibles, respectively. The glasses are melted and the temperature is then loweredquickly to the drawing temperature whereupon fiber fabrication commences.
Figure 6
Canes
Assembly of canesInsert canes into jacket
Preform
Piston
Sleeve
Billet
Preform
(a) Stack-and-draw
(c) Extrusion
Rolling of polymer on a rod Preform
(b) Thin-film-rolling
Three main strategies to produce IRMOF preforms: (a) stack-and-draw, (b) thin-film rolling, and (c) extrusion methodologies. The photograph in (c) is of anextrusion die that produces a six-holed preform.
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The fiber outer diameter and the core–cladding ratio are controlled by adjustingthe gas pressures in the inner and outer tubes independently, by altering thedrawing rate or by adjusting the temperature [155–158]. As such, both multi-mode and single-mode fibers can be drawn using this process. Since the fiber isdrawn directly from the melt, the two glasses are required to have closer vis-cosities than tolerable when drawing from a preform. This constraint usuallyexcludes the possibility of achieving a large-refractive-index contrast. The dou-ble crucible method typically provides higher-quality fibers compared to thosedrawn from a rod-in-tube preform, but the latter offers a higher level of controlover the fiber structure and dimensions.
IR fibers may also be fabricated by the hot-extrusion technique, which enables“drawing” polycrystalline halides [118,119,124,159] into fibers with diametersin the range 500–900 μm with no buffer jacket. In this method, a single-crystalbillet is placed in a heated chamber and a piston forces it through a die. Step-index fibers have been co-extruded into fiber in this fashion [118,159], but theyusually have a highly irregular core region and poor core–cladding interfacequality, resulting in higher losses than unclad fibers. There has been muchprogress in reducing the loss in clad polycrystalline IR fibers through carefuladjustment of the core and cladding compositions and the extrusion parameters.To date, Ag-halides produce the best polycrystalline IR fibers [124].
Crystalline materials are not amenable to thermal fiber drawing without supportfrom a glassy backbone or scaffold material forming an outer cladding
Figure 7
Pressure
(a)
Fiber
Furnace
Core
Cladding
(b)
Preform
Sleeve
Billet
Die
Two layers of deposited semiconductors
(c)
Hollow fibers
Precursors, dopants By-products
High temperature environment
Non-preform-based approaches to produce IR fibers: (a) the double cruciblemethod, (b) hot extrusion, and (c) HPMCD.
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(Sections 3 and 8). Nevertheless, crystalline sapphire fibers have been producedvia modified crystal-growth techniques in which the fiber is pulled from the meltusing edge-defined, film-fed growth or laser-heated pedestal growth techniques[160,161].
More recently, HPMCD [74] has emerged as a microscale variant of chemicalvapor deposition (CVD) to produce fibers from materials not amenable to ther-mal drawing, particularly single-crystal semiconductors including ZnSe [17](Section 8). The main drawback of this technique is the limited lengths of fiberproduced compared to thermal drawing.
5. Heavy Metal Oxide Glass Infrared Fibers
Heavy metal oxide glasses (HMOGs), such as germanate, lead-silicate, and tel-lurite glasses, have—in general—spectral transmission windows that extendover the VIS and MIR (Section 2). Furthermore, these glasses are endowed withbetter mechanical and thermal characteristics and possess, in addition, higheroptical nonlinear coefficients than fluoride glasses (Section 6). Nevertheless,they have lower nonlinearities and shorter transmission cut-off wavelengthscompared to chalcogenide glasses (Section 7) [Fig. 8(c)]. Fibers made of theseglasses have been commercialized [23] and are finding increasing applicationsfor laser gain material.
5.1. Tellurite Glass Infrared Fibers
The tellurite glass family was discovered by Stanworth in 1952 [162], and thefirst significant characterization of their optical properties in fiber form was re-ported in 1994 [63]. Tellurite glasses offer a transmission window that extendsfrom the VIS to the MIR [Fig. 8(c)]. In the 1990s, rare-earth-ion-doped telluriteglass fibers attracted attention as potential NIR broadband amplifiers for tele-communications applications [163–165] and recently for white-light generationthrough nonlinear upconversion [166]. Over the past decade, the high opticalnonlinearity of tellurite glasses [63,167,168] has attracted interest for applica-tions in IR nonlinear optical processing [169–172]. Critically, from the perspec-tive of fiber drawing, the glasses exhibit high crystallization stability relative tofluoride glasses, so they may be readily shaped into a large variety of preformstructures using casting [164,173,174], drilling [175], and extrusion techniques[176]; see Fig. 9.
The key to capitalizing on the intrinsic MIR transmission of tellurite glass is thereduction of OH groups, which cause strong absorption at 3–4 μm. Severalmethods, including melting in a dry atmosphere, raw-material dehydration,
The most common oxide glass is based on silicates, which are notuseful in fibers transmitting IR light. Replacing silicon with heavymetals (e.g., lead and tungsten) results in oxide glasses with theirtransparency window extending further into the IR, albeit not as faras fluoride and chalcogenide glasses.
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and the use of fluoride or chloride raw materials and combinations thereof,have been investigated to reduce the OH content in tellurite glasses, and recentresults are listed in Table 3 [164,173–184]. The OH content in tellurite glasses iscaused by water impurities in the raw materials. There is an equilibrium betweenthe water vapor in the atmosphere above the glass melt and the water in theglass melt (in the form of OH groups). Use of a dry atmosphere releases thewater from the melt to the atmosphere. Churbanov et al. [177] found thatthe OH content in glass is proportional to the square root of water vapor pressureover the melt. The release of water from the melt increases with longer meltingtime and higher melting temperatures [176,177]. However, melting timeand temperature are limited to prevent significant evaporation of the glass meltitself.
In addition to the use of a dry atmosphere, partial replacement of oxide rawmaterials by fluoride raw materials has been demonstrated to be an effectivemethod to reduce the OH content [174,175,178,179,181,183–185].Unfortunately, fluorotellurite glasses exhibit reduced crystallization stability
Figure 8
Photographs of typical (a) sodium-zinc-tellurite (b) and lead-germanate glassbillets. Scale bars are 20 mm. (c) Absorption spectra of sodium-zinc-tellurite(TZNL) and lead-germanate (GPLN) glasses melted in ambient and dry atmos-pheres. Reprinted with permission from [111]. Copyright 2013 Optical Societyof America.
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and have lower linear and nonlinear refractive indices, which is undesirable fortarget nonlinear applications such as supercontinuum generation. Furthermore,fluoride incorporation reduces the glass transition temperature, which leads toreduced thermal and mechanical stability. As an alternative to fluoride, the use ofchloride raw material has been shown to be an effective dehydration agent [164].
Apart from OH groups, metal impurities such as 3d and 4f elements cause ab-sorption and thus enhanced loss. This effect is particularly prominent in theNIR, where absorption due to OH groups is significantly lower compared withthe MIR. Table 3 lists the results of fiber loss measurements in the NIR (1.5–2.1 μm). The use of commercially available, high-purity raw materials (99.999%and higher for TeO2, 99.99% and higher for other raw materials) led to lossesof 0.1–0.2 dB/m at 1.55 μm [175,176]. The use of ultrapure raw materials madein-house has been demonstrated to result in bulk glass losses of 0.04–0.08 dB/mat 1.56 μm and 0.05–0.10 dB/m at 1.97 μm [174,179]. Single-index fibers madefrom such ultrapure glasses exhibited higher losses of 0.5 and 0.3 dB/m at 1.56and 1.97 μm, respectively [174]. However, step-index multimode fibers madefrom such ultrapure glasses demonstrated losses that were similar to the bulkglass loss: 0.05 dB/m at 1.6 and 2.1 μm [179]. The lowest NIR loss was reportedfor a tellurite fiber made from in-house prepared TeO2 is 0.02 dB/m at 1.55 μm[187]. Recently, multimode tellurite fibers produced by NP Photonics, Inc. haveshown no measurable OH absorption at 3–4 μm, and a minimum loss of0.2 dB/m at 3.5 μm [23].
For tellurite glass fibers with low OH content, it becomes apparent that the multi-phonon edge is composed of two components with different slopes [111]. Theshort wavelength tail of the multiphonon edge in the range of approximately
Figure 9
Thermal drawing Extrusion
Extruded preform Cane
Jacket tube
Fiber
Preform
(a)
(b)
Steps to fabricate a small-core extruded tellurite preform. (a) Preform prepara-tion; (b) resulting tellurite MOF [186].
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4.0–5.7 μm has a smaller slope, whereas for wavelengths >5.8 μm a steep edgeis observed. The short wavelength tail causes losses of ∼10–500 dB∕m in therange of 4.0–5.7 μm, which presents a severe limitation for MIR fiber applica-tions. For bulk glass samples with usually a few millimeter to ∼1 cm thickness,the short wavelength tail has a negligible or small impact on the transmission,and thus the steep multiphonon edge >5.8 μm limits the transmission of bulkglass samples.
In conclusion, the combination of high nonlinearity, IR transmission up to 4 μm,high rare-earth solubility, high crystallization stability, and relatively goodchemical durability make tellurite glasses attractive candidates for fiber lasersand nonlinear optical applications in the IR. However, the relatively high lossdue to OH groups at 3–4 μm and minimum loss of >0.1 dB∕m in most fibersdemonstrated to date has hampered commercial applications of tellurite glass IRfibers.
5.2. (Lead)–Germanate and Tungsten–Tellurite-GlassInfrared Fiber
Germanate glass fibers generally do not contain fluoride compounds. Theyalso do not contain silica (SiO2); rather they contain heavy metal oxides toshift the IR absorption edge to longer wavelengths. The advantage of germanatefibers over fluoride fibers is that germanate glass has a higher Tg and, therefore,a higher laser-damage threshold, but the loss for the fluoride fibers islower.
Within the tellurite glass family, Na, Li-Zn-tellurite glasses have been mostwidely investigated for supercontinuum generation [169,175,194,195] and IR
Table 3. Example of Recently Synthesized Tellurite Glasses and Fibers with Low OHContent
a X � O, F2, Cl2.b For bulk glass or fiber at ∼3.3–3.4 μm.c Not available.
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laser applications at wavelengths above 2 μm [196–198]. However, the low Tg ∼300°C combined with high thermal expansion coefficient (TEC) of∼�190–200� × 10−7∕°C for these glasses (Table 4) limits their thermal andmechanical stability, resulting in low laser damage threshold, in particularfor small-core tellurite fibers [194]. Therefore, recently, lead–germanate glasses[111,187,191,197] and tungsten–tellurite glasses [189,199] have been investi-gated as viable alternatives to Zn-tellurite glasses. Both glass types exhibit com-paratively high Tg of ≥400°C and low TEC of �110–120� × 10−7∕°C, whilehaving high refractive indices of 1.9–2.1 similar to Zn-tellurite glasses(Table 4). Lead–germanate glasses have the advantage of lower phonon energiescompared with tungsten–tellurite glasses (Table 4), which is of importance forapplications in the MIR. For applications that do not require high refractive in-dices, e.g., high-power fiber lasers at ∼2 μm, Ba-Ga-germanate glasses havebeen demonstrated to be an attractive alternative [192,200]. This type exhibitshigh Tg of>600°C, which is accompanied with lower refractive index, while thephonon energy is between those of lead–germanate and tungsten–telluriteglasses (Table 4).
6. Fluoride Glass Infrared Fibers
Fluoride glasses exhibit the lowest refractive index among IR glasses, hence, thelowest optical nonlinearity [64,167,201], which makes them particularly wellsuited for high-power delivery and lasing applications where nonlinear effectsare undesirable. Another unique property of fluoride glasses is their extremelybroad transmission window from the UV (∼300 nm) to the MIR (4–6 μm for∼1-m-long fiber) [64,167,201,202]; see Fig. 10.
Fluoride glasses can be divided into four types: fluoroaluminate glasses based onAlF3, fluorozirconate glasses based on ZrF4, fluoroindate glasses based on InF3,
Summary. (Lead–)germanate and tungsten–tellurite glasses offer IRtransmission up to ∼4 μm, relatively high nonlinearity, and rare-earth solubility combined with higher thermomechanical stabilitycompared to zinc–tellurite and fluoride glass fibers. This makessuch glasses particularly well suited for applications requiring highoptical nonlinearity and gain combined with high laser-damagethreshold. To date, these characteristics have been exploited in fiberlasers at ∼2 μm.
Table 4. Example of Typical Tellurite and Germanate Glasses Used for High-Nonlinearity and High-Power Applications in the Infrareda
Glass Composition Tg °C TEC 10−7∕°C n Eph (cm−1) Ref.
a Tg, glass transition temperature; TEC, thermal expansion coefficient; n, refractive index at1–1.5 μm; Eph, phonon energy.
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and fluoride glasses based on divalent fluorides. As the energy of the stretchingvibration between the metal and fluorine ions decreases in the order AlF3 >ZrF4 > InF3 > MF2, the MIR transmission edge is shifted to longer wave-lengths in the same order [64,202,203].
The most-established and widely used fluoride glasses are the fluorozirconateglasses. The first ZrF4-based glass was discovered in the 1970s [41]. Within thisglass family, the so-called “ZBLAN” glass with composition (in mol. %)53ZrF4–20BaF2–4LaF3–3AlF3–20NaF has been the most widely used.Indeed, ZBLAN exhibits high crystallization stability, enabling low-loss fiberfabrication [64,204]. By contrast, no fibers have been reported for divalent-fluoride-based glasses, which is attributed to their low crystallization stability.Recently, fluoroindate glasses have gained increased interest due to their ex-tended transmission compared with ZBLAN [22,202], while fluoroaluminateshave not gained comparable interest for MIR applications due to their limitedtransmission up to ∼4 μm.
6.1. ZBLAN Glass Fibers
For fluorozirconate glasses, it was found that the theoretical loss limit of<0.01 dB∕km at 2–3 μm is 1 order of magnitude lower than that of silica[205]. This discovery stimulated a large amount of research in the 1980s and1990s to develop ultralow-loss fluorozirconate fibers. Although a low loss of0.7 dB∕km at 2.7 μm was demonstrated [206,207], the intrinsic loss limit wasnot achieved due to extrinsic losses caused by metal impurities and scattering
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To date, fluoride glasses—compared to other IR glasses—have metwith the most commercial success in fibers produced for IR powerdelivery and active fiber applications, such as fiber lasers.Nevertheless, the transparency window of fluoride glasses does notextend beyond 5.5 μm.
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defects (mainly small crystals) in the glass [64,207]. The use of high-purity rawmaterials (both metals and fluorides) is essential to reduce both extrinsic absorp-tion (through a reduction in their water and oxide content) and scattering losses(by preventing the formation of crystals in the glass). Unfortunately, ultrahigh-purity, ultradry fluoride raw materials are not commercially available and thusin-house purification is required to fabricate ultralow-loss fluoride glass[204,208]. In addition to raw-material purity, glass processing conditions playa significant role. Numerical simulations demonstrated that extreme fiber draw-ing conditions of high draw speed and high tension can reduce fiber loss to levelsclose to the theoretical intrinsic loss [209]. However, these extreme drawingconditions are not practical.
Building on the large effort in developing low-loss fibers, fluorozirconate step-index fibers with 5–50 dB∕m at 2–3 μm are commercially available [64].Recently, it was demonstrated that MOFs can also be produced fromZBLAN glass using extrusion or the stack-and-draw technique [210,211].
ZBLAN glass fibers have found widespread interest in fiber lasers [37,212–214]and supercontinuum generation applications [215–218]. The combination of lownonlinearity, MIR transmission up to 5 μm, and low optical loss make ZBLANparticularly well suited for high-power lasing and beam delivery applications. Arange of rare-earth-doped ZBLAN fiber lasers operating in the MIR at 3–4 μmhave been demonstrated [37,213]. High-power delivery requires large fibercores combined with good beam quality, which cannot be provided by tradi-tional multimode fibers. However, the MOF technology was demonstrated toenable both large mode area and high beam quality [210]. Although ZBLANglass exhibits low nonlinearity, its interest for supercontinuum generation (anonlinear process) stems from its low loss in the MIR at 3–6 μm [219–221]and zero dispersion wavelength at ∼1.6 μm, which is close to the wavelengthsof available high-power erbium-doped fiber lasers at 1.5 μm and thulium fiberlasers at ∼2 μm [216]. Kubat et al. proposed an approach for generating MIRsupercontinuum by using concatenated fluoride and ChG fibers pumped with apulsed thulium fiber laser [222]. To date, investigation of supercontinuumgeneration has been limited to step-index ZBLAN fibers [218]. A ZBLANMOF with tailored dispersion has been recently demonstrated [211].
Figure 11(a) shows a photograph of multiple pathways toward producing fluo-ride glass fiber preforms. One example is a cast rod that be drawn directly, whileanother example includes cast disks that are then extruded 20°C above the glasstransition temperature (Tg � 310°C) using graphite dies into rods for fiberdrawing. One preform (Preform 1) was cleaned by isopropyl alcohol in an ultra-sonic bath prior to fiber drawing, while the other preform (Preform 2) was etchedin a 15 wt. % HCl solution to remove a ∼0.5-mm-thick outer layer. Both cast rodand extruded rods were then pulled into unclad fibers with diameters of∼130–180 μm. Furthermore, the extrusion method has also been used to pro-duce ZBLAN MOFs [Fig. 11(b)] [210].
6.2. Fluoroindate Glass Fibers
As noted above, fluoroindate glasses offer extended IR transmission due to theirlower phonon energies. Moreover, they offer higher Tg of ∼300°C [64,202]compared to ZBLAN glass, whose Tg ∼ 260°C [203,204], thereby promisinghigher thermal stability.
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The potential for extended IR transmission stimulated research on the develop-ment of fluoroindate fibers in the past decade [64,223,224]. Step-index fluoroin-date fiber with low loss of 0.6 dB∕m at 5 μm is now commercially available[22]. At this wavelength, the theoretical loss of ZBLAN is 6 dB∕m (calculatedusing the equations and parameters given in [167]). Recently, extruded fluoroin-date fibers with 2 dB∕m at 5 μm, made from commercially available rawmaterials, have been reported [202].
Figure 11
(a) Fluoroindate glass in the form of (left) cast billets, (top) cast rod, (center)preforms extruded from cast billets. Reprinted with permission from [110]Copyright 2013 Optical Society of America. (b) Extruded preform and scanningelectron microscope image of a ZBLAN MOF Reprinted with permission from[210] Copyright 2008 Optical Society of America.
Summary. Fluoride glass fibers offer IR transmission up to 5–6 μmcombined with low optical nonlinearity. They are particularly wellsuited for applications requiring high-power handling and lowoptical loss. Therefore, fluoride glasses are the material of choicefor fiber lasers in the range 3–5 μm. Recently, fluoride glass fibershave also attracted increasing interest for supercontinuumgeneration applications. The drawbacks of fluoride glasses are theirrelatively poor chemical stability, requiring the fibers to be wellpackaged or used in a dry environment. In addition, fluoride glassesexhibit low crystallization stability compared with oxide andchalcogenide glasses, which hinders their processing into fiberswith complex structures. ZBLAN glass is the most widely usedfluoride glass for IR fiber applications. Although fluoroindate glassoffers extended IR transmission, its higher crystallization tendencyduring fiber drawing hampers development of low-loss fibers.
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7. Chalcogenide Glass Infrared Fibers
Chalcogens are the chemical elements in group VIA of the periodic table—specifically the elements sulfur (S), selenium (Se), and tellurium (Te).Chalcogenide glasses (ChGs) are endowed with a unique set of physical char-acteristics that have made them attract considerable recent interest despite theirlong history [66]. From the perspective of their utility in IR fibers, ChGs are wellknown for their broad IR transparency—extending to the FIR—and amenabilityto thermal drawing. Typical commercially available ChG compositions arebased on As-S(Se), Ge-As-Se(Te), As-Se-Te, Ga-La-S, and Ge-Sb-Se systems[42,108,225].
Bulk samples that are millimeter-thick of S-, Se-, and Te-based ChG transmitlight in the 0.5–12 μm, 1–16 μm, and 1.5–20 μm spectral windows, respectively,as shown in Fig. 12. In addition to their IR transparency, ChGs have the highestthird-order nonlinear refractive indices among all optical glasses. These charac-teristics make ChGs ideal candidates for MIR nonlinear fiber optics where shortfiber lengths or ultralow optical power levels are sufficient to elicit nonlinearoptical behavior. The typical optical loss in ChG fibers in the MIR is of theorder of 0.1–10 dB/m [120,157,226–228]. These values are much higher thantheoretical predictions that indicate a minimum attenuation of 11 dB/m at 4.5 μmfor GeS3 [205] and ∼0.01 dB∕m loss at 5.0 μm from As2S3 ChG fibers [229].Therefore, despite substantial efforts over the past several decades, the full po-tential of ChGs has not yet been achieved.
Although the concentration and mobility of free charge carriers in ChGs arelower than in crystalline semiconductors and the Fermi level is apparentlypinned, the combination of these optical and electronic characteristics in anIR glass bodes well for the development of novel functionalities in ChG fibers.Indeed, ChGs—despite their amorphous structure that lacks long-range order—are p-type semiconductors, unlike all IR glasses described in the previous
Figure 12
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(a)–(c) Photographs of three typical S, Se, and Te ChGs [342]: (a) As2S3,(b) As2Se3, and (c) Ge20As20Te45Se15. (d) Typical IR transmission spectra(starting at 1 μm) of S, Se, and Te ChG millimeter-thick bulk samples.
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sections that are electrical insulators [230]. Furthermore, ChGs exhibit thresholdand memory-switching phenomena [231] not known to exist in other glass sys-tems. Consequently, besides their utility in IR fibers, ChGs have been exploitedrecently in fabricating waveguide devices for MIR sensing [70,232,233], non-linear optics [234,235], integrated photonics [71,236], laser amplifiers[237,238], and ultrahigh-bandwidth optical signal processing [239,240].Furthermore, CVD techniques or thermal evaporation are used to fabricateChG-based waveguides [241,242] or even 1D MOFs [243].
Efforts on multiple fronts have been aimed at developing active MIR sourcesbased on ChG fibers. One strategy relies on the IR nonlinear properties ofChG fibers [43,44,169244–256], and there has been success in demonstratingChG Raman fiber lasers [257–263]. Another approach has been to use ChGs ashosts for rare-earth ions (REIs) [218,256,264–299], or nanoscale doped crystal[300–302]. Despite the extensive research on REI-doped ChGs since the 1990s,there has been only limited success to date [34,37]. Further efforts have beencarried out on identifying ChG compositions that enable high doping concen-trations [272,273,303] while maintaining low phonon energy for IR emission,rare-earth co-doping schemes [266,304], in addition to reducing the fiber loss byincreasing the ChG purity and uniformity [305]. Crystalline Cr2�: ZnS∕Senanoparticles have also been introduced into AsS-Se glass systems and fibersfor active applications [300–302]. More details on the major applications of ChGfibers are provided in Refs. [228,256,306].
ChG IR fiber sensors have proven to be excellent candidates for real-time remotequantitative detection and quantification of gas, organic, and biological species[307–317]. Low-phonon Te-based ChGs show great potential for analyzing theatmosphere of extra-solar Earth-like planets in search of life as their wide opticaltransmission spectrum encompasses the spectral signatures of H2O (∼6 μm), O3
(∼9 μm), and CO2 molecules (∼16 μm). To further cover the FIR region, newTe-based ChGs have been developed with transparency windows extending upto 25 μm [226,318–320].
The systematic study of ChGs as IR materials started in the mid-20th century,dating back to the investigation by Frerichs of As2S3 glass [321]. Subsequently,As-S step-index ChG fibers were reported in 1965 by Kapany and Simms[322], who demonstrated a relatively high transmission loss of 20 dB/mat 5.5 μm.
Considerable efforts by several Japanese corporations and agencies laid thegroundwork for future developments in ChG fibers, including the NipponTelegraph and Telephone Public Corporation (NTT) [157,323,324], Hitachi,Ltd. [325–329], Horiba, Ltd. [330], Non-oxide Glass R&D Co., Ltd. [331],the Kyota Semiconductor Corporation [332], the HOYA Corporation [333],
Uniquely, ChGs cover the broadest IR spectrum of all IR glasses, havethe highest optical linear and nonlinear refractive indices, and offer thewidest tuning range of optical parameters achieved throughcompositional engineering. ChG fibers may potentially cover theentire span of QCL wavelengths.
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and the Communications Research Laboratory (now The National Institute ofInformation and Communications Technology, NICT) [334,335]. These effortsled to drawing fibers from several ChG systems (Ge-P-S, As-S, As-Ge-Se, Ge-S,Ge-As-Se-Te-(Tl),), having either unclad or step-index [157,331,336] structures,in addition to fiber bundles [329,334,337], where each strand in the bundle con-sists of a ChG core and a Teflon cladding. The mechanical robustness of ChGfibers was fortified by developing UV-curable polymer coatings or incorpora-tion of a Teflon FEP polymer cladding at the preform level—thereby prescientlyinitiating the field of multimaterial fibers [10]. Furthermore, the study of opticalnonlinearity [338–341] and REI doping in ChG fibers [333] were initiated.These extensive efforts established the framework for future developmentsand highlighted the need for efficient methodologies to produce low-cost,low-loss, robust ChG fibers.
In the USA, Amorphous Materials, Inc., started the manufacturing of bulkChGs in 1977, and subsequently reported the fabrication of ChG fibers and fiberbundles using several compositions from the GeAsSe, AsSe, AsS, and AsSeTesystems—the most successful being As-Se-Te (2–11 μm) and As2S3 (VIS–8 μm) glasses. Despite the lack of mechanical robustness that plagues these fi-bers, they were used to transmit watt-level (<5 W) of CO and CO2 CW laserlight through fibers with cores of hundreds of micrometers. The IR imaging fiberbundles were based on As2S3 fibers with a plastic epoxy serving as cladding,which limits the IR transmission spectra. See the interesting book [72] thatrecounts the detailed history of these efforts.
Research and development efforts in the area of ChG fibers have now prolif-erated around the world. Examples of such teams include The Institute ofChemistry of High-Purity Substances of the Russian Academy of Sciences(1980s–), Université de Rennes 1 (1990s–), the US Naval ResearchLaboratory (NRL) (1990s–), University of Southampton (1990s–), and theUniversity of Nottingham (1990s–), which have collectively led to the currentmaturation of the ChG fiber field.
7.1. Current Status of Optical Losses in ChalcogenideGlass Fiber
Bulk ChGs are normally prepared by melt quenching from high-purity(99.999%–99.9999%) elements and compounds that may have incongruentmelting points, exhibit high partial vapor pressure during melting, and are po-tentially susceptible to oxidation and hydrolysis. Therefore, synthesis must becarried out in sealed evacuated quartz ampules in the absence of oxygen orwater. ChGs need to be agitated to promote mixing and homogeneity duringthe melt-based processing as the elemental constituents react to form a glassliquid. The primary contaminants ([O], [H], [C], and dissolved compounds)—which have a noticeable impact on the properties of the drawn optical fibers—may largely be attributed to trace-level constituents in the starting raw materials[343]. Consequently, attempts to obtain ChGs via alternate routes, such as theuse of nonvolatile compounds, have been made [225]. The expected advantageof such an approach is a lower rate of impurity inclusion, if synthesis is possibleat a lower temperature over a shorter period of time. In addition to the melt-quenching technique, several other approaches have been developedto produce glassy ChGs, such as the utilization of microwave radiation
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[344–346] or CVD [347]. Despite the wide range of bulk ChGs available, only asubset of thermally stable glasses has been found useful for thermal fiberdrawing.
The lowest optical attenuation to date in ChG fibers [120] remains approxi-mately ×1000 higher than the intrinsic losses (estimated to be 0.08 dB/km at5.0 and 6.1 μm for As-S and As-Se glass fibers, respectively [348]).Extensive investigations by Russian researchers over the past few decades intothe nature and origin of impurities in ChGs [343,349–355] have led to a reduc-tion in optical loss <1 dB∕m in fibers from the As-S, As-Se, and As-S-Sesystems (except for several IR absorption bands caused by stubborn impurities[356]). Using the double-crucible method, step-index As-S fibers were fabri-cated in the early 1990s [158], followed by step-index As-Se-Te and As-S-Seglass fibers with minimal optical losses of 0.15 dB/m at 6.6 μm and 0.06 dB/m at4.8 μm, respectively [357]. To date, the lowest loss for ChG-based optical fibershas been achieved in multimode As2S3 optical fibers, with losses of 0.012 and0.014 dB/m at 3.0 and 4.8 μm, respectively [120].
Since the late 1980s, scientists at Université de Rennes 1 have developed Te-based ChGs (Te-ChG) fibers, especially from the Te-halide (TeX, X � Cl, Br, I)systems [358,359]. Both unclad and step-index fibers have been produced (bythe rod-in-tube and double-crucible methods) that offer wider transmission win-dows than S- and Se-ChGs, typically up to 9–9.5 μm [360–363]. Indeed, 2.6 Woutput at 9.3 μm power was obtained from 7 W input power out of a 1-m-long,600-μm-diameter unclad TeX fiber provided with an antireflection coating[363]. The long-wavelength transmission Te-ChG fibers has enabled their usefor remote chemical analysis/detection and temperature sensing [364–366]. Theminimum optical loss of unclad Te-ChG fibers (TeAsSe system) is less than0.1 dB∕m in the 6.7–7.3 μm window [307], while step-index single-modeTeAsSe fibers have a minimum loss of typically ∼0.33 dB∕m at 7.5 μm[367]. Broadband step-index Te-ChG fibers that possess higher loss(7–40 dB∕m in the 4.0–15.0 μm region) have been produced by the rod-in-tubeapproach for the Darwin mission [368].
The U.S. Naval Research Laboratory started in the 1990s to report their efforts indeveloping IR ChG fibers from the As-S, As-Se-S, As-Se-Te, and Ge-As-Se-Tesystems [156,228,229,256,369–373] in a variety of structures, including uncladand step-index fiber and MOFs. The core and cladding diameters of a typicalsingle-mode ChG fiber fabricated by the double-crucible technique are 12 and125 μm, respectively, with the addition of a 62.5-μm-thick acrylate coating andtypical loss ∼0.9 dB∕m at 2.7 μm for As-S(Se)-based fiber [374]. The step-index ChG IR fibers that have emerged from these efforts are currentlycommercialized [24,25].
7.2. Enhancement of Mechanical Robustness
Polymer/plastic/resin coatings (typically UV acrylate) have been used to offermechanical protection to fragile ChG fibers for several decades [39]. Suchprotection is critical for ChG fibers, which possess only 1/10 the tensile strengthof silica glass fibers [375]. To overcome this drawback, several approacheshave been explored, including coating the fiber with a combination of multiplepolymer layers [376], providing a jacket by heat-shrink that reduces the inevi-table interfacial gap usually remaining when the rod-in-tube approach is used
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[373], and more recently including a thick built-in thermomechanically compat-ible jacket at the preform level [13,122,123] surrounding a step-index ChGstructure through multimaterial coextrusion—all of which result in bettermechanical support when compared to the traditional single-layer polymercoating.
The general idea of combining glasses with polymers in an optical fiber has beeninvestigated since the 1980s (typically via the rod-in-tube approach) to appro-priate the favorable mechanical properties of polymers and compensate for theless-favorable mechanical properties of soft glasses. Thermal co-drawing ofglasses with a polymer jacket, such as Teflon, results in fibers with greatly im-proved mechanical properties—whether step-index fluoride [377] or ChG [157]fibers, or IR fiber bundles [329,334,337]. However, there are inherent limita-tions imposed on the dimensions of the internal glass structure and the outerdiameter that stem from reliance on a single fiber-drawing step. Furthermore,lack of independent control over the dimensions of the glass and polymer por-tions of the fiber leads to thermal effects, limiting the power handling capabilityof the fiber. These dimensional limitations are lifted using the multimaterialcoextrusion fabrication approach.
The brittleness of ChGs sets limits on the machining of such glasses for preformpreparation and also on the strength of drawn ChG fibers. Indeed, the fragilityand difficulty of handling and processing of ChG IR fibers have limited theirwidespread use [379]. Despite impressive progress, there has been no definitiveanswer to the lack of robustness of ChG fibers. Recent efforts have culminated inthe development of a low-cost process that yields robust multimaterial ChG fi-bers with broad dimensional control over both the ChG and the polymer sectionsof the multimaterial fiber [13,122,123,380]. Additionally, the diversity of appli-cations of ChG fibers—stemming from the very distinct optical, electronic, andoptoelectronic characteristics of these materials—requires a fabricationapproach flexible enough to harness material combinations with precisedimensional control not usually attainable through the rod-in-tube or double-crucible approaches.
A recent addition to the IR fiber fabrication portfolio has been that of multima-terial coextrusion combined with thin-film-rolling processes, as shown inFig. 13. A first generation of this process utilized a vertically stacked billetof ChG and polymer disks that is extruded through a small die, leading tothe transformation of the vertical disks into a cylindrically nested structure[Fig. 13(a)].
A variation (or “second-generation”) on this theme [123] exploits a structuredextrusion billet that minimizes the amount of glass needed to produce anIR fiber, as shown in Fig. 13(b)—leading to high-efficiency “disc-to-fiber”coextrusion. As alluded to above, large-scale synthesis of high-purity ChG—necessary for the usual pathways to producing ChG fibers—remains a materials-processing challenge, especially in an academic environment, and thus presentsan obstacle to the transfer of research results from academia to industry. Themodified billet structure shown in Fig. 13(b) allows for the drawing of∼50 m of robust IR ChG fiber starting from only ∼2 g of glass! This approachwill hopefully enable rapid prototyping of ChG fibers from the wide range of
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available compositions tailored with specific applications in mind without thestringent requirements of large-scale, high-purity ChG synthesis. Finally, thismultimaterial coextrusion approach is sufficiently flexible to extend to a widespan of ChG compositions, including Te-ChGs coextruded from a multimaterial
Figure 13
(a)–(c) Three multimaterial coextrusion strategies that differ in the billet struc-ture. (d) Producing a preform using the thin-film rolling technique [13,122,123].(e) Photograph of extended lengths of drawn multimaterial ChG fibers [378].(f) Reflection optical micrographs of the fiber cross section. G1, As2Se3; G2,As2S3; P, polyethersulfone (PES) [13]. (g) Photograph of a robust ChG multi-material nanotaper. (h) Transmission spectrum of a robust Te-ChG multimaterialfiber [122].
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rod-in-tube billet [Fig. 13(c)] [122]. Figure 13(h) is the transmission spectrum ofa 3-cm-long Te-ChG fiber sample [122].
When using any of the three coextrusion variations in Figs. 13(a)–13(c),the extruded rod is then moved to a thin-film-rolling step followed by thermalconsolidation under vacuum to produce a preform [10]. This extra step providesflexibility over the relative dimension of the ChG and polymer sections withoutadding further constraints on the coextrusion process. Robust IR fibers are thenthermally drawn in an ambient atmosphere into continuous lengths of fiber withdesired diameters. Furthermore, multiple draws or additional stack-and-drawsteps may be used to control the ultimate size of the fiber core [381].
The multimaterial ChG fibers resulting from this procedure have multiple salu-tary features that may be exploited in a variety of settings: (1) dramatic increasethe mechanical robustness of the fiber [13], (2) enabling one to take advantage ofthe wide IR transmission window of ChGs, (3) increasing the potential refractiveindex contrast between fiber core and cladding (controllable from 0.02 to >1),which is useful in dispersion and nonlinearity engineering [43,44,244], (4) en-abling control over the core-to-cladding diameter ratio [13], (5) control over thedimension of the core from millimeters down to a few nanometers [381], and(6) reduction of the volume of costly IR material [13]. Several of the advantagesof these multimaterial ChG fibers combine to deliver novel nonlinear fiber de-vices. Since the polymer and the ChG are thermally compatible, the fibers maybe tapered without first removing the polymer, leading to robust tapers even withsubmicrometer core diameters [43,44,244]. The large index contrast affordedbetween the core and cladding allows for both dispersion control [244] andstrong field confinement in the core [13], which allows one to overcome thehigh material group velocity dispersion (GVD) of ChGs and at the same timeharness their high nonlinearities (Table 1).
By adapting the new concept of multimaterial fibers to bear upon the stubbornproblem of lack of mechanical robustness in IR fibers, IR-transparent ChGs maybe exploited despite their inferior mechanical characteristics by combining themwith thermoplastic polymers to produce robust multimaterial IR fibers. Thisoverall strategy, developed mainly at CREOL, The College of Optics &Photonics (University of Central Florida), may pave the way to a new generationof robust ChG IR fibers.
Silica-based MOF technology typically relies on arranging a lattice of air holesin an otherwise solid cladding. ChGs of various compositions uniquely offer alarge range of refractive indices ranging from 2.1 to 3.5 in the MIR, thereby
Recent innovations in multimaterial fiber fabrication technology inwhich thermoplastic polymers are combined with ChGs yieldrobust, continuously drawn, extended lengths of fiber, with widetunability of the geometric and physical parameters. This approachmay render ChG fibers commercially viable and useful for QCLlight transmission across the entire IR.
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offering the opportunity for all-solid MOFs that nevertheless maintain large in-dex contrasts that are not accessible in silica. In addition, ChG MOFs with airholes in the cladding have also been demonstrated. Several fabrication ap-proaches have been exploited to produce ChG MOFs, including stack-and-draw[18,275,382,383], mechanical drilling [135], rod-in-tube [384], and castingmethods [148]. Initial theoretical [385] and experimental [20] studies of hol-low-core ChG bandgap MOFs have been realized, but more work is requiredbefore conclusive PBG guidance is demonstrated. Furthermore, suspended-coreChG fibers have been demonstrated, with applications in supercontinuum gen-eration [250,386,387].
8. Multimaterial Infrared Fibers
The past decade or so has witnessed a proliferation of innovative approachesfor producing fibers from materials or material combinations that are nottypically associated with optical fibers. For example, while both polymersand glasses have each individually been utilized to draw fibers, combiningpolymers and glasses in the same fiber has only been recently explored[7,10,11,21,32,122,123,388]. As another example, while crystalline semicon-ductors are the mainstay of the electronics and optoelectronics industries, theiruse in optical fibers has not been considered except very recently. In both theseexamples, issues related to the thermomechanical compatibility of the materialssystems with traditional fiber drawing have been conceived as a hindrance tosuccess, and conventional wisdom has thus proven here to be an unwelcomeroadblock.
In Section 3, we have seen that careful design of the fiber preform can helpalleviate some traditional constraints in material choices with regards to fiberdrawing. Several successful multimaterial fibers now bear witness to the fruitful-ness of this general approach to producing IR fibers [10]. An early example isthat of hollow-core fibers where IR light is guided via a PBG effect—a 1D peri-odic photonic structure endowed with high index contrast lines the core andconfines the light via an omnidirectional reflection effect [388]. The high indexcontrast needed for successful hollow-core guidance is achieved by a uniquecombination of optical polymer (low index) and soft glass (high index) thatare, nevertheless, thermomechanically compatible at the drawing temperature.These fibers have laid the foundation for a new generation of IR fiber deliverydevices for minimally invasive medical surgery, and have to date helped save orimprove thousands of lives [33].
On a different front, the IR transparency of crystalline semiconductors(Section 2) have invited efforts to draw semiconductor-core, glass-cladding fi-bers [389–391] with the amorphous cladding facilitating the drawing procedure(Section 3). Several semiconductors have been drawn continuously in this fash-ion, ranging from elemental semiconductors such as Si [391] and Ge [79] tocompounds such as InSb [16]. These early achievements now require concertedefforts to tackle the challenges that face this paradigm: identifying thermome-chanically compatible amorphous cladding materials that are IR-transparent,elimination of unwanted thermochemical reactions, and reduction of irregular-ities along the fiber stemming from the mismatch of thermal expansion coeffi-cients of the crystalline core and the glassy cladding. Success in this effort may
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usher in a new synthesis of optics and electronics—the two main information-processing technologies of our time.
Finally, recent efforts have been directed at the use of hollow silica fibers as a“substrate” or scaffold for deposition of single-crystal semiconductors from avapor phase [74]. Although this approach may not produce the fiber lengthstypically resulting from thermally drawing a preform, useful active fiber devicesmay be produced. For example, IR-transparent ZnSe [17], a wide-gap II–VIsemiconductor [392] that finds many optical and optoelectronic applications,has been deposited via CVD in the hollow core of a silica fiber while maintain-ing high-quality polycrystallinity. It is conceivable that such fibers may lead tonovel IR fiber lasers at wavelengths extending beyond 2 μm [393].
In this section, we elaborate on these three families of multimaterial fibers. Inaddition to the other examples described in the previous sections (such as thehybrid robust ChG-polymer fibers in Section 7), it is clear that the field of multi-material fibers is in a healthy growth phase that promises many surprises in theyears to come [10].
Silica optical fibers—the mainstay of the telecommunications and otherindustries—rely on index guiding of light through a solid core and, thus, havefundamental limitations in their attenuation and nonlinearities that stem from theinteraction of light with a dense, solid core. Since the MIR is particularly chal-lenging for fiber transmission owing to the high absorption losses in mostglasses and polymers used in fiber optics, hollow-core fibers offer the oppor-tunity to greatly reduce these limitations. One of the earliest efforts on suchfibers for the MIR relied on thermal drawing of hollow-core fibers lined withan all-dielectric omnidirectional reflecting mirror [32,388,394]. Light is con-fined to the fiber core by a large PBG established by a high-refractive-index-contrast multilayer stack comprised of a high-index chalcogenide glass and alow-index polymer [21,395–402]. The layer dimensions determine the transmis-sion bandgap, which can be tuned from the VIS to the IR. Light propagationthrough air in a hollow fiber greatly reduces problems associated with materialabsorption, nonlinearities, thermal lensing, and end reflections, and facilitateshigh-power laser guidance and other applications that may be impossible usingconventional solid-core fibers.
Early notions of hollow-core fibers lined by a multilayered reflective surfacehave existed since the 1970s [403], but there are multiple challenges associatedin realizing such structures. First, a pair of materials must be identified that—onthe one hand—have compatible thermomechanical properties to enable them tobe co-drawn at the same temperature and—on the other hand—have a high re-fractive-index contrast. Second, in order to prevent scattering and obtain low-loss fibers, the drawing process must preserve the interfaces of the multilayerstructure down to the microscale. Figure 14(a) depicts the fabrication procedureused to produce such fibers. To date, several pairs of materials have beenidentified that yield low-loss, hollow-core, multimaterial PBG fibers under ap-propriate fabrication conditions. A typical pair consists of a low-refractive-indexpolymer and a high-refractive-index ChG, glassy materials that are thermome-chanically compatible for fiber drawing [21,395,396] (Section 3).
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Figure 14(b) shows a typical hollow-core fiber structure and layer uniformityresulting from the fiber draw. The desired transmission PBG can be achievedby controlling the fiber draw speed, which, in turn, directly scales the layer thick-nesses. Figure 14(c) shows the transmission spectrum of a fiber drawn to trans-mit in a wavelength range centered at the CO2 laser wavelength of 10.6 μm, withlosses below 1 dB∕m. This is 1 order of magnitude less than the intrinsic lossesof the chalcogenide glass (As2Se3) and 5 orders of magnitude less than the pol-ymer (PES) that make up the multilayer reflector. These relatively low losses aremade possible by the fact that most of the energy is carried in the hollow coreand by the very short penetration depth of the core-guided fiber modes in thePBG structure [398], allowing these materials to be used at wavelengths thatwould be considered impractical in the index-guiding regime.
Figure 14
Multimaterial PBG fibers. (a) Schematic of the multimaterial PBG fiber fabri-cation process. (i) The high refractive-index-contrast in the layered structure lin-ing the core is achieved by thermal evaporation of a ChG (As2Se3, refractiveindex of 2.8) onto a thermoplastic polymer (PES, refractive index of 1.55).(ii) This bilayer film is subsequently rolled onto a mandrel to form the multilayerstructure and additional polymer cladding films are added for mechanical sta-bility. (iii) The entire structure is thermally consolidated under vacuum until thematerials fuse together into a solid preform. (iv) After removing the mandrel, thecross-sectional dimensions of the preform are reduced by drawing the preforminto a fiber. The ratio of the preform down-feed speed and fiber draw speeddictates the final layer thicknesses. (b) Cross-sectional SEM micrograph of ahollow cylindrical multilayer fiber mounted in epoxy. The hollow core appearsblack, the PES layers and cladding are gray, and the As2Se3 layers are brightwhite. The PES layers are 900 nm thick and the As2Se3 layers are 270 nm. Thisfiber has a fundamental PBG centered at 3.55 μm. (c) Typical transmission spec-trum of hollow-core fibers designed to transmit CO2 laser light. The fundamentalPBG is centered near a wavelength of 10.6 μm, and the second-order PBG is at5 μm. Inset: plot of the logarithm of the transmitted power versus the fiber lengthreveals a loss of 0.95 dB∕m.
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Hollow-core multimaterial PBG fibers have already gained a solid footing in themedical device industry, where they are used for the delivery of high-power CO2
laser light in minimally invasive surgical procedures [33]. Future research in thisarea is expected to focus both on new materials and process development forfurther reducing transmission losses, as well as exploration of new applications,such as chemical vapor sensing, where hollow core fibers can be employed as amedium that can simultaneously transmit light and various chemical species fordetection and analysis [404–410].
Optical fibers comprising cores of semiconductor materials have gained consid-erable attention [411] given their potential to unify two fields central to moderninformation and computing technologies: silicon photonics [412] and fiber op-tics. Given the rapid growth of interest in this topic, this section will summarizethe state of the art. For completeness, the reader is referred to several thoroughreviews of semiconductor optical fibers for greater detail [80,413].
Semiconductor materials as core phases in optical fibers pose a unique oppor-tunity for optical device, component, and system designers. Unlike conventionalsilica glass or less-conventional soft-glass IR materials, semiconductors possessa range of optical and optoelectronic functionality [411] that could be of greatvalue as growing demands continue to be placed on photonic systems. For ex-ample, cubic semiconductors can possess very large Kerr optical nonlinearitiesthat are useful for efficient wavelength conversion and optical signal processing[413]. To date, only cubic semiconductor phases have been realized in opticalfiber form and losses have generally been too high to make practical devices,with the exception of recent work employing microspheres generated using asemiconductor optical fiber [414]. Indeed, if the technologies can be maturedsuch that crystalline χ�2� semiconductor-based fibers can be fabricated, thenthe field of nonlinear fiber optics could be redefined.
To date, two principal fabrication approaches have emerged—each possessingrelative advantages and disadvantages. The first employs CVD of unary andselected binary semiconductors inside of silica microstructured optical fibers[73]. The main advantages of this approach are that both amorphous and crys-talline semiconductors cores can be realized, as can optoelectronic junctions be-tween differently doped semiconductors [26,415], realized, however, at slowgrowth rates in short fiber lengths that these produced by thermal drawing(see below).
The second main approach is the molten-core method, which has been employedto make glass-clad optical fibers with crystalline cores of Si [391], Ge [79],and InSb [16] (see Fig. 15). In the molten-core method, the core phase is selectedsuch that it is molten at the temperature where the cladding glass draws intofiber; see Ref. [416] for a recent review of this process. For example, Si (Tmelt ∼1414°C) is sleeved inside a pure silica (T draw ∼ 1950°C) tube and drawn directlyinto fiber. The advantages of this method are long lengths (>100 s meters tokilometers) and compatibility with existing commercial optical fiber fabricationinfrastructure. Disadvantages include the necessary high draw temperatures tofabricate fibers with silica cladding, which promotes thermochemical reactions
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that can dissolve silica (from the cladding glass) into the semiconductor core,leading to scattering from precipitated oxide phases.
From a practical perspective much optimization remains with respect to bothfabrication methodologies. Optical losses at present are high (∼dB∕cm) andthe attainment of closer-to-theoretical values (10s of dB/km [80]) requires con-tinued effort. That said, the dominant sources of fairly high propagation loss areunderstood and are being addressed systematically [417].
From a materials perspective, only oxide-based cladding glasses have been em-ployed to date for all of the semiconductor optical fiber efforts. In the IR, wheresuch fibers hold their greatest potential for applications, the oxide cladding willlimit performance by absorbing the evanescent field of the propagating mode.This is especially the case for single-mode and small-diameter (e.g., tapered[418]) semiconductor-core fibers. IR-transparent claddings have been identified[419] and are presently under development. Furthermore, and for completeness,purity and scattering issues associated with oxide contamination from the clad-ding glass are being addressed through creative use of diffusion barriers and insitu melt-phase chemistry. While there likely is no “one size fits all” solution tothe optimization of future semiconductor optical fibers, a combination of de-signer cladding glasses with diffusion/buffer layers and precursor purificationmeasures could rapidly reduce the present attenuation values (dB/cm) to thelevels (dB/m) where practical devices become possible.
MIR optical fibers are typically not as robust as silica optical fibers [124]. Theconstraints on viscosity placed by the fiber drawing process limit the palette ofmaterials suitable for these fibers primarily to those that have relatively low op-tical damage thresholds and strength [124]. MIR optical materials such as crys-talline compound semiconductors (e.g., zinc chalcogenides) can have muchhigher optical damage thresholds but, in general, cannot be drawn into fibersalone because they have sharp melting points. Many of them also melt incon-gruently or have high melt vapor pressures that make drawing impossible [17].Oxidation, deviations in stoichiometry, and the incorporation of impurities areadditional issues that must be addressed when drawing such fibers.
Sparks et al. fabricated ZnSe and ZnS optical fibers by high pressure CVD overcentimeters of length into silica capillaries [Fig. 16] and silica MOF [Fig. 17(a)]
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[17,73,392]. High pressures facilitate mass transport into the silica fiber pores[74]. The silica cladding into which the ZnSe fiber cores are deposited allowsthese fibers to have high strength and be handled much as conventional silicafibers. Losses thus far for ZnSe fibers with single cores 10s of micrometers indiameter range from 0.5 to 1 dB∕cm. The wavelength dependence of the opticalloss varies as λ−3.9, suggesting that grain boundary or other bulk inhomogeneityscattering may be dominant. Chalcogenide fibers can be much longer than crys-talline ZnSe fibers and can exhibit much lower optical losses of the order of1 dB∕m, but are limited to continuous wave optical power densities of 10 to20 kW∕cm2 [124], much lower than that of silica.
Compound semiconductor fibers are anticipated to have much higher opticaldamage thresholds than soft-glass fibers in view of their refractory nature, highthermal conductivity, and wide optical bandgaps [73]. The ability of crystallinecompound chalcogenide semiconductors such as ZnSe to host transition metalions, e.g., Cr2�, that have excellent optical gain properties in the MIR suggeststhat one of the first applications of ZnSe fibers may be for high-power tunableMIR fiber lasers. ZnSe fiber lasers should have less thermal lensing and im-proved thermal management in comparison with bulk ZnSe lasers. Fibers fab-ricated from ZnSe may also be useful for nonlinear frequency conversion inview of its substantial second-order nonlinear optical coefficient.
Figure 16
ZnSe optical fiber. A centimeters-long, 50 μm diameter, cylindrical ZnSe opticalfiber core, embedded in a silica cladding, which is not visible, is illuminated withVIS light. Inset: optical micrograph of a smaller-diameter ZnSe optical fiber.
Figure 17
(a) Optical micrograph of core–cladding ZnSe microstructured fiber fabricatedfrom a silica fiber template. (b) Finite element simulation of the fundamentalmode for this fiber at 1550 nm. (c) Finite element simulation of the second-ordermode at 1550 nm. Inset in (b) shows the measured near-field intensity of theguided mode at 1550 nm.
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Lower optical losses, greater control over mode structure, and longer length fi-bers can be anticipated in the future for chemically deposited semiconductorfibers. Microstructured ZnSe optical fibers allow for control over mode geom-etry [Figs. 17(b) and 17(c)]; these modes overlap with only a small amount ofsilica, decreasing losses due to silica absorption. Deposition of a Zn-S/Se clad-ding layer followed by a ZnSe core can allow for complete elimination of themode overlap with silica present in silica clad ZnSe fibers (Fig. 10) [392].Appropriate modification of the grain/microstructure may allow for considerablylower optical losses, as the intrinsic optical loss of ZnSe is very low [73]. Finally,the deposition of Si layers up to 10 m long [415] inside silica fiber pores suggeststhat longer chemically deposited optical fiber cores may be possible in thefuture.
9. Other Infrared Fibers
In view of the difficulty of identifying IR-transparent materials that produce low-loss and robust IR fibers, a long-exploited approach has been to rely on hollow-core fibers, the earliest realization being hollow metal fibers [420] (see Section 8for hollow-core multimaterial PBG fibers as an alternative avenue). Such fibersoffer various advantages; for example, they may transmit wavelengths well be-yond 20 μm, in addition to having low insertion loss, minimal end reflection, andsmall beam divergence. In contrast to solid-core IR fibers, their hollow-corecounterparts have high damage thresholds for high-peak-power and high-average-power lasers (over a kilowatt of CW laser power in Ref. [421], forinstance). Indeed, it has been noted that front-end clipping in a CO2 laser beamdelivered through a hollow-core metal fiber is the main cause of thermal load-ing [422].
9.1. Hollow-Core Silica Infrared Fibers
While silica is of itself quite opaque in the MIR, the robustness of silica and thewell-established technology of silica fiber fabrication have motivated the searchfor photonic structures that nevertheless allow for low-loss MIR propagationutilizing silica. The key feature of this approach is to create structures that min-imize the fraction of light guided in the material to take advantage of the IRtransparency of air or other gases in the core. Indeed, a hollow-core silicaPBG fiber demonstrated MIR transmission in 2005 [423] with a loss of2.6 dB∕m at a wavelength of 3.14 μm, which is 2 orders of magnitude lowerthan the material loss in silica at this wavelength. With a 40-μm-diameter core,this fiber exhibited quasi-single-mode guidance and low bend losses.
Summary. Combining multiple materials with disparate optical,electronic, and mechanical characteristics in the same fiber—so-called multimaterial fibers—is a research endeavor that hasflourished over the past decade and is currently having a significantimpact on IR fibers. This methodology is particularly useful inproducing fibers from IR materials that cannot be drawn into afiber directly, such as crystalline semiconductors.
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Recent efforts have further reduced the fraction of light overlapping with thesilica glass by exploiting a hollow-core design with negative curvature alongthe circumference of the hollow core. While hollow-core PBG fibers rely onPBG guidance from a periodic cladding structure, negative curvature (NC) fibersor antiresonant (AR) fibers rely for guidance on a combination of inhibited cou-pling to low density of states cladding modes and antiresonance [424,425]. Thetypical advantages of AR fibers over PBG fibers are expanding new transmis-sion regions in the UV and IR spectral regions, while also offering wider band-widths than PBG fibers. This concept originates from the early studies onantiresonant reflecting optical waveguides (ARROWs) in SiO2-Si multilayerstructures in the 1980s [426]. More studies were carried out on the guiding con-ditions, single-mode operation, and analysis of leakage properties of variousARROWs where the core is either a low-refractive-index material or air (hollowcore) [427–433]. The major motivation behind these studies is the applicationsof these waveguides in integrated photonics, sensing, and quantum communi-cations. Similar simple design strategies were applied to cylindrical waveguides(fibers) consisting of high-index inclusions that surround a low-index core, ei-ther to integrate them with existing fiber infrastructures or to extend the trans-mission to longer wavelengths into the MIR and THz region both theoreticallyand experimentally [434–443]. The demonstration of high-average-power pico-second and nanosecond pulse delivery at the NIR (1030 and 1064 nm, respec-tively) region [444] and MIR (2.94 μm) [54] through hollow-core AR fibersproves the potential for applications in micromachining and surgical devices.Furthermore, deep-ultraviolet (UV) light is also guided by a double AR hol-low-core fiber in the single-mode regime by modified tunneling of leaky modes[445]. This design led to fibers with losses of 0.034 dB∕m at 3.05 μm wave-length for a 9.4 μm core diameter [446] and 50 dB∕m at 7.7 μm for a 119 μmcore [434]. These fiber losses are 3–4 orders of magnitude lower than those ofbulk silica at these wavelengths. See Table 5 for a comparison of the state of theart. These fiber examples demonstrate the potential of the hollow-core fiber ap-proach to achieve MIR guidance at wavelengths where the fiber material hassignificantly high loss.
The limitation of this approach is that larger core diameters (>10 μm) are re-quired to reduce the fraction of guided light in the material surrounding thehollow core. Furthermore, hollow-core fibers, in general, suffer from somedrawbacks, ranging from the need for more complicated fabrication methodol-ogies to difficulties in splicing to other fibers. Additionally, surface modes ap-pear in these structures, limiting significantly the fiber properties. Recent effortsto experimentally reduce the bending loss of hollow AR fibers [447] and theo-retically explore new design strategies for ultralow loss at the MIR region [448]further prove the feasibility of utilizing AR fibers with multiple applications andpositive aspects for future commercialization. Interestingly, the hollow-core ARfiber approach has been successfully extended to IR glasses, such as ChGs, thatare inherently IR transparent. A recent report demonstrated a fiber with11 dB∕m loss at 10.6 μm for a 380-μm-diameter core [449].
9.2. Hollow Metallic Infrared Fibers
Fabrication of such fibers typically involves the use of a glass, polymer, or met-allic tubing in which metal and dielectric layers are deposited to enhance theIR reflectivity from the inner surface [450–452]. Miyagi et al. pioneered the
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development of metallic waveguides based on a hollow nickel substrate [451],and further developed dielectric coatings (ZnS) over silver with losses as low as0.25 dB∕m at 10.6 μm [453], in addition to hollow-core structures having asquare cross section with 0.1 dB∕m loss at 10.6 μm [454]. A critical disadvant-age of hollow metallic fibers is the high bending losses (which depends largelyon the quality of the inner surface).
To date, hollow metallic fibers have found numerous applications in transmittingbroadband IR light for dental and medical laser treatment and industrial lasermaterials processing. Moreover, metallic IR fibers are an ideal platform for ther-mal radiometry (the peak of blackbody radiation near room temperature isaround 10 μm) and have been used to transmit radiation produced in the non-destructive measurements of jet engine blade temperatures (corresponding toblackbody radiation above 1000°C). More recently, hollow metallic fibers havebeen used to transmit incoherent light for broadband spectroscopic and radio-metric applications [455–457]. In addition they may be used as delivery systemsin chemical remote sensing applications, either as a passive fiber or as an activeplatform filled with the medium to be probed [458]. Nevertheless, these fibershave not been fully accepted as flexible delivery systems for industrial lasers,due partially to the relatively high loss when compared to other technologies(e.g., articulated arms) and to the potential distortions of beam profiles inlarge-diameter multimode hollow-core metal fibers.
9.3. Crystalline Infrared Fibers by Hot Extrusion
Step-index polycrystalline alkali halide fibers have been fabricated using a hot-extrusion technique [119,459]. An example is KBr/KCl fibers, which have dem-onstrated a minimum loss of ∼0.1 dB∕m at 10.6 μm [119]. There are run-to-runvariations in fabricating such fibers, leading to an average loss 0.69 dB∕mwith astandard deviation of 0.32 dB∕m over different runs (varying typically in therange between 0.3 and 1.0 dB∕m) for freshly extruded step-index fibers withcore/cladding diameters of 800/1000 μm. The maximum output power fromsuch a fiber is 67 W of CW CO2 laser power, corresponding to a power densityof 13.3kW∕cm2. Bending the fiber with a diameter of curvature of 12 cm wasfound to reduce the transmission through such 1000 μm salt fibers by ≈5%.These fibers are typically coated with Teflon to minimize surface fracture frommicrocleavage cracks and to protect the fiber from contamination.
Recently, multiple hot extrusion of flexible 1-mm-diameter, 1-m-long silverhalide polycrystalline fibers has been reported [460]. These fibers are highly
Table 5. Examples of Hollow-Core Fiber with Low Loss in the MIR
Fiber Type Dcore (μm) Material λ (μm)Material Loss
(dB/m)Fiber Loss(dB/m) Ref.
PBG via omnidirectionaldielectric mirror
700 ChGpolymer
10.6 101 for glass 105
for polymer0.95 [21]
PBG via periodic air/glassstructure
40 silica 3.14 6 × 101 2.6 [423]
ARG via negative-curvaturehollow core
94 silica 3.05 6 × 101 0.034 [446]
ARG via negative-curvaturehollow core
119 silica 3.4 7.7 6 × 101 105 0.05 50 [434]
ARG via negative-curvaturehollow core
380 silica 10.6 5 × 101 11 [449]
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transparent in the MIR. The core was AgBr while the cladding consisted or AgClfiber optic elements arranged in two concentric hexagonal rings around the core.These (effectively step-index) fibers are potentially useful for IR laser powertransmission, IR radiometry, and IR spectroscopy [435,461–463].
9.4. Hybrid Infrared Fibers by Pressure-Assisted MeltFilling
ChG–silica hybrid fibers of limited length (a few centimeters) may be fabricatedusing a pressure-assisted melt-filling technique, as illustrated in Fig. 18. First, aChG fiber is inserted into a silica capillary with inner diameter 150 μm and outerdiameter 200 μm [Fig. 18(a)]. This capillary is then spliced to a second capillaryhaving inner and outer diameters of 1 and 200 μm, respectively. The inner diam-eter of the second capillary determines the resultant ChG core diameter. Second,both capillaries are placed in an oven at a high temperature (∼600°C for As2S3glass) while applying argon gas to force the melted ChG from the first capillaryinto the channel of the second [Fig. 18(b)]. A filling time of typically 1 h isneeded for ChG to fill in a few-centimeter length. Small bubbles that nucleateand grow during the filling process may be removed using a heat source scannedalong the capillary. Finally, a continuous part is sectioned and cleaved to formthe target short fiber.
This technique has been successfully adapted to S- and Se-ChG and telluriteglasses to fabricate hybrid fiber with core diameters ranging from 200 nm to6 μm [14,464], with the aim of broadband MIR supercontinuum generation.Most recently, this technique was modified to integrate nanotapers at the inputand output ends of the fiber to increase the incoupling and outcoupling effi-ciency [465]. In addition to step-index fibers, it is also possible to fabricate hy-brid MOFs using this technique to realize all-solid bandgap guidance [466,467].
Figure 18
Silica MOF fiber (~ µm core)
Silica capillary (~ ×100 µm core)
Splicing
Heat zone
Pressure
(a)
(b)
Soft-glass
Soft glass in a silica MOF
Pressure-assisted melt-filling technique for ChG-core, silica-cladding IR fiber.(a) A soft glass fiber is inserted into a silica capillary, which is spliced withanother silica capillary. (b) High temperature and pressure are applied to forcesoft glass into the channel of the left silica capillary.
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10. Future Prospects
As mentioned in the introduction, considerable recent research in optics has beenmobilized by the prospect of extending well-known concepts and technologiesfrom the VIS and NIR to the MIR. This less-explored spectral vista offers uniquecapabilities, particularly in metrology, sensing, and biomedicine. Consequently,the field of IR fibers—as an essential technology for facilitating the utilization ofthe IR spectrum—is currently witnessing sustained and dramatic growth.Perhaps a measure of one aspect of this growth, at least from the perspectiveof academic research, is the number of published journal articles and citations,as shown in Fig. 19. Steady growth is readily observed, especially with regardsto tellurite and chalcogenide fibers. Figure 19 obviously relates to only a seg-ment of the field of IR fibers; therefore, these figures represent a lower estimate,but are nevertheless indicative of the field in general. The concomitant growth inthe IR fiber industry, as evinced by recent commercialization of IR fibers forQCL light delivery, also bodes well for future growth.
Figure 19
1980 1990 2000 20100
50
100
150
200
250
No.
of
publ
icat
ions
Years
Chalcogenide Fluoride Tellurite Germanate
1980 1990 2000 20100
1000
2000
3000
4000
5000
6000
No.
of
cita
tions
Years
Chalcogenide Fluoride Tellurite Germanate
(a)
(b)
Number of publications and citations to these publications at Web of Sciencefrom 1980 to 2014. The search was carried out using the following keywords inthe title: “X AND glass AND fiber,” where X corresponds to “Chalcogenide,”“Fluoride,” “Tellurite,” or “Germanate,” as shown in the figure legend.
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Several critical tasks lie ahead for the IR fiber community. There is acute needfor standardization: in fabrication processes (especially of bulk IR glasses) andin characterization techniques—and even in nomenclature—to facilitate the ex-change of knowledge across the various research communities involved in op-tical physics and engineering and materials science. Since there appears to be noone-size-fits-all material or fabrication approach that will dominate this field inthe foreseeable future, it will be crucial to develop more applications that benefitfrom IR light to continue to motivate research.
This review may also help lay out a roadmap for future developments in IR-fiber-based technologies that will be critical for exploiting the IR spectrum.The first milestones, which have already been partially addressed, lie in theoptimization of IR fibers for the delivery of QCL light and also high-powerIR lasers. Both of these optical sources are undergoing rapid developmentand the availability of convenient IR delivery fibers will have profound impacton applications in biosensing, pollution monitoring, and defense. Furthermore,novel fiber-based IR sources are a potential growth area. There has beentremendous progress in IR supercontinuum generation from IR fibers[14,43,44,169,216,220,246,248,250,468–473], but other nonlinear wavelengthconversion schemes need also to be developed, such as Raman shifting and four-wave mixing. The high optical nonlinearity of IR glasses and the potential fordispersion engineering in high-index-contrast fibers may be exploited in produc-ing IR laser combs [474–476]. Moreover, postprocessing of IR fibers, via taper-ing, for example, can allow for tailoring the fiber device characteristics to atarget function [254,477–480]. Finally, while research on REI doping of IR fi-bers has occupied the community for decades, there has not yet been a decisivebreakthrough that may lead to high-power lasing using the approaches followedin shorter-wavelength (VIS and NIR) fiber lasers [37].
In achieving the milestones along this roadmap, the technical hurdles facing IRfibers—such as high optical losses and mechanical fragility—need to be at thefore of the community’s attention. Hurdles to the standardization of proceduresfor fiber facet polishing, fiber splicing, and connectorization need to be tackled.Additionally, recent progress in spatial mode control and modal analysis will beof utility in both linear and nonlinear applications of IR fibers [481]. Suchadvances will particularly facilitate extending the capabilities of fiber sensingtechnology to the IR.
We anticipate a bright future for IR fibers and hope that this article serves as anentry point to the vast literature that has already accumulated in this field.
Acknowledgments
G. Tao acknowledges Dr. Rongping Wang and Dr. Shangran Xie, for illuminat-ing discussions; Dr. Shangran Xie, Prof. Philip St. J. Russell, Dr. Fei Yu, andProf. Jonathan Knight, Dr. Jas Sanghera, Dr. Mohammed Saad, Prof. James A.Harrington, Dr. Tomer Lewi for offering images of their results; and He Ren, Dr.Kunlun Yan, Ruilin Zheng, Dr. Yinsheng Xu, and Dr. Xinghua Yang for assis-tance with preparing this manuscript. H. Ebendorff-Heidepriem acknowledgesJiafang Bei for glass transmission measurements. S. Danto acknowledges Prof.Thierry Cardinal for his support. Work at the University of Adelaide was sup-ported by the Defence Science and Technology Organisation (DSTO), the AsianOffice of Aerospace R&D (AOARD), the South Australian Government, and the
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Australian Research Council (ARC). Work at the University of Adelaide wasperformed in part at the OptoFab node of the Australian NationalFabrication Facility utilizing Commonwealth and SA State Government fund-ing. Work at Pennsylvania State University was supported by the U.S. NationalScience Foundation (DMR-0806860, DMR-1107894, and NSF MRSEC DMR-0820404). Work at MIT was supported by the Materials Research Science andEngineering Program of the U.S. National Science Foundation (DMR-0819762)and also in part by the U.S. Army Research Office through the Institute forSoldier Nanotechnologies (W911NF-07-D-0004). Work at ClemsonUniversity was supported by Northrop Grumman Corporation and theRaytheon Company. Work at the University of Central Florida was supportedby the U.S. National Science Foundation (ECCS-1002295) and in part by theU.S. Air Force Office of Scientific Research (FA-9550-12-1-0148).
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Guangming Tao received his B.Eng. in 2006 fromShandong University, M.Sc. in 2009 from FudanUniversity, and Ph.D. in 2014 from the University ofCentral Florida in optics/optoelectronics. He was a visitingscholar at the Chinese Academy of Science (2007, 2008) andthe Massachusetts Institute of Technology (2012) and an in-frared material engineer at LightPath Technologies (2012,2013). He is a Research Scientist at CREOL, The College
of Optics & Photonics, the University of Central Florida. He has been awardedseveral scholarships/honors from The Optical Society (OSA), the InternationalSociety of Optical Engineering (SPIE), and the American Ceramic Society(ACerS), and was awarded the Extraordinary Potential Prize of the ChineseGovernment Award for Outstanding Self-financed Students Abroad (2013).Dr. Tao is also a cofounder and CTO of Lambda Photonics LLC (2014),an IR fiber startup company that developed out of his Ph.D. work. He has yearsof research experience in sciences and engineering in academia, industry, andgovernment institutes with expertise in the areas of infrared material, infraredfiber and fiber laser, in-fiber nanofabrication, in-fiber energy devices, and novelsmart fibers with unique functionalities.
Heike Ebendorff-Heidepriem received the Ph.D. degreein chemistry from the University of Jena, Germany, in1994, where she continued her research on optical glassesuntil 2000. During 2001–2004 she was with theOptoelectronics Research Centre at the University ofSouthampton, UK, working on novel photosensitive glassesand soft-glass microstructured optical fibers with record highnonlinearity. Since 2005, she has been with the University of
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Adelaide, Australia. Currently, she is one of the leaders of the Optical Materials& Structures Theme at the Institute for Photonics & Advanced Sensing at TheUniversity of Adelaide. She was awarded the Woldemar A. Weyl InternationalGlass Science Award and a prestigious EU Marie Curie Individual Fellowshipin 2001. Her research focuses on the development of mid-infrared, high-nonlinearity and active glasses; glass, preform and fiber fabrication techniques;and surface functionalization of glass.
Alexander M. Stolyarov received a Ph.D. in AppliedPhysics from Harvard University in 2012. His doctoraland postdoctoral research was conducted in the group ofProf. Yoel Fink at MIT, where he worked on novel multi-material fiber structures, including microfluidic fiber lasers,liquid crystal fiber devices, chemical sensing fibers, andhollow-core photonic bandgap fibers for high-power lasertransmission. Alexander has coauthored 15 journal publica-
tions and holds two U.S. patents. Currently, he is a member of the technical staffin the Chemical, Microsystem, and Nanoscale Technologies Group at MITLincoln Laboratory.
Sylvain Danto received his Ph.D. in Materials Science fromthe University of Rennes (France) in 2005 where he workedon tellurium-based glasses for data storage and infraredoptics. Next he joined the group of Prof. Y. Fink at MIT,where he developed the in-fiber glass/crystal phase-changecapability for fiber-based optoelectronics, and then the groupof Prof. K. Richardson at Clemson University in 2011. He iscurrently working at the University of Bordeaux, where he
continues his research on glasses for applications in optics and photonics.
John V. Badding is a professor in the ChemistryDepartment and the Department of Physics & Astronomyat Pennsylvania State University. He received his doctoratefrom U.C. Berkeley in 1989. After postdoctoral work inhigh-pressure science as a Carnegie Fellow at theGeophysical Laboratory of the Carnegie Institution ofWashington, he moved to Penn State in 1991. He visited theOptoelectronics Research Center at the University of
Southampton for a year in 2001. His research interests are in the area of materialschemistry relevant to fields such as photonics, carbon nanomaterials, semicon-ductor nanomaterials, and polymers. His group has pioneered the high-pressurechemical vapor deposition approach to the fabrication metal and semiconductoroptical fibers for applications such as solar fabrics, high-speed in-fiberjunction detectors, and very-high-power infrared fibers and lasers. Both unarysemiconductors and compound semiconductors that are difficult to draw in pureform can be deposited by this method. He is the author of 190 publications andhas received both NSF and David and Lucile Packard Foundation fellowships.
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Yoel Fink is a professor of materials science and engineeringand Director of the Research Laboratory of Electronics (RLE)at theMassachusetts Institute of Technology (MIT). ProfessorFink received a B.Sc. in Chemical Engineering (1994) and aB.A. degree in Physics (1995) from the Technion, Haifa. In2000hewas awarded aPh.D. degree inMaterials Science fromMIT. That same year, he joined the faculty of the MITMaterials Science and Engineering Department, and in
2011 he became a joint professor of electrical engineering and computer science.He was a recipient of the Weizmann Institute Amos De-Shalit FoundationScholarship in 1992, was awarded the Hershel Rich Technion InnovationCompetition in 1994, was a recipient of the Technology Review Award for the100 Top Young Innovators in 1999, and was awarded the National Academy ofSciences Initiatives in Research Award for 2004. In 2006 he won the Joseph LaneAward for Excellence in Teaching, and in 2007 was named one of the MITMacVicar Fellows, an award given in recognition of outstanding teaching abil-ities. Professor Fink’s research group has pioneered the field of multimaterialmultifunctional fibers. His research focuses on extending the frontiers of fibermaterials from optical transmission to encompass electronic, optoelectronic,and even acoustic properties. Professor Fink is a co-founder of OmniGuideInc. (2000) and served as its chief executive officer from 2007 to 2010; he is cur-rently an active member the Board. He is the coauthor of over 75 scientific journalarticles, and holds 44 issued U.S. patents on photonic fibers and devices.
John Ballato is Vice President for Economic Developmentat Clemson University. A professor of materials scienceand engineering and electrical and computer engineering,he founded and directed for 14 years the Center forOptical Materials Science and Engineering Technologies(COMSET). Dr. Ballato has published 300 archival scientificpapers, holds over 25 U.S. and foreign patents, has givenin excess of 150 keynote∕invited lectures, and has
co-organized 70 national and international conferences and symposia. He hasbeen a Principal Investigator on more than $46 million worth of sponsored pro-grams. Among numerous other honors, he is a Fellow of The Optical Society(OSA), the International Society of Optical Engineering (SPIE), and theAmerican Ceramic Society (ACerS).
Ayman F. Abouraddy received the B.S. and M.S. degreesfrom Alexandria University, Alexandria, Egypt, in 1994 and1997, respectively, and the Ph.D. degree from BostonUniversity, Boston, Massachusetts, in 2003, all in electricalengineering. In 2003 he joined the Massachusetts Institute ofTechnology (MIT), Cambridge, as a postdoctoral fellowworking with Prof. Yoel Fink and Prof. John D.Joannopoulos, and then became a Research Scientist at
the Research Laboratory of Electronics in 2005. At MIT he pursued researchon novel multimaterial optical fiber structures, photonic bandgap fibers,
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nanophotonics, fiber-based optoelectronic devices, and mid-infrared nonlinearfiber optics. He is the coauthor of more than 60 journal publications and130 conference presentations, holds seven patents, and has three patents pend-ing. He joined CREOL, The College of Optics & Photonics, at the University ofCentral Florida as an assistant professor in September 2008, where he has sinceestablished facilities for fabricating new classes of polymer and soft-glass fibersfor applications ranging from mid-infrared optics to solar energy concentration.
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